diff --git a/apps/edr-freight-api/src/migrations/3550000000000-EimsDebitCreditNotes.ts b/apps/edr-freight-api/src/migrations/3550000000000-EimsDebitCreditNotes.ts
new file mode 100644
index 000000000..cf36df757
--- /dev/null
+++ b/apps/edr-freight-api/src/migrations/3550000000000-EimsDebitCreditNotes.ts
@@ -0,0 +1,28 @@
+import { MigrationInterface, QueryRunner } from "typeorm";
+
+/**
+ * Debit/credit note filing — confirmed directly by MoR support: same `/v1/register` endpoint,
+ * distinguished by `DocumentDetails.Type` ("DEB"/"CRE") + a `Reason`, linked to the original
+ * invoice via `ReferenceDetails.RelatedDocument`. See `Invoice.eimsDocumentType`.
+ */
+export class EimsDebitCreditNotes3550000000000 implements MigrationInterface {
+  name = "EimsDebitCreditNotes3550000000000";
+
+  public async up(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query(`
+      ALTER TABLE freight.invoices
+      ADD COLUMN IF NOT EXISTS eims_document_type varchar(8) NOT NULL DEFAULT 'INV',
+      ADD COLUMN IF NOT EXISTS eims_reason text,
+      ADD COLUMN IF NOT EXISTS related_invoice_id uuid REFERENCES freight.invoices(id)
+    `);
+  }
+
+  public async down(queryRunner: QueryRunner): Promise<void> {
+    await queryRunner.query(`
+      ALTER TABLE freight.invoices
+      DROP COLUMN IF EXISTS eims_document_type,
+      DROP COLUMN IF EXISTS eims_reason,
+      DROP COLUMN IF EXISTS related_invoice_id
+    `);
+  }
+}
diff --git a/apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.spec.ts b/apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.spec.ts
index 927e2efec..7406613fc 100644
--- a/apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.spec.ts
+++ b/apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.spec.ts
@@ -212,6 +212,59 @@ describe("toEimsInvoice", () => {
     expect(() => toEimsInvoice(invoice({ issuedAt: null }), seller, context())).toThrow(/not issued/);
   });
 
+  describe("debit/credit notes — confirmed by MoR support, same /v1/register endpoint", () => {
+    it("defaults DocumentDetails.Type to INV with no Reason field", () => {
+      const doc = toEimsInvoice(invoice(), seller, context());
+      expect(doc.DocumentDetails.Type).toBe("INV");
+      expect(doc.DocumentDetails).not.toHaveProperty("Reason");
+    });
+
+    it("files a credit note with Type, Reason and RelatedDocument", () => {
+      const doc = toEimsInvoice(
+        invoice(),
+        seller,
+        context({
+          documentType: "CRE",
+          reason: "Overbilled freight charge",
+          relatedDocument: "9fe9bbbece6ab76c112b617534e6aac7aa8b819d5be79f4d3d088ed2e887b2e0",
+        }),
+      );
+      expect(doc.DocumentDetails).toMatchObject({ Type: "CRE", Reason: "Overbilled freight charge" });
+      expect(doc.ReferenceDetails.RelatedDocument).toBe(
+        "9fe9bbbece6ab76c112b617534e6aac7aa8b819d5be79f4d3d088ed2e887b2e0",
+      );
+    });
+
+    it("files a debit note the same way", () => {
+      const doc = toEimsInvoice(
+        invoice(),
+        seller,
+        context({ documentType: "DEB", reason: "Additional handling fee", relatedDocument: "IRN-1" }),
+      );
+      expect(doc.DocumentDetails).toMatchObject({ Type: "DEB", Reason: "Additional handling fee" });
+    });
+
+    it("throws when a credit/debit note has no reason", () => {
+      expect(() =>
+        toEimsInvoice(
+          invoice(),
+          seller,
+          context({ documentType: "CRE", reason: null, relatedDocument: "IRN-1" }),
+        ),
+      ).toThrow(/needs a reason/);
+    });
+
+    it("throws when a credit/debit note has no relatedDocument", () => {
+      expect(() =>
+        toEimsInvoice(
+          invoice(),
+          seller,
+          context({ documentType: "CRE", reason: "Overbilled", relatedDocument: null }),
+        ),
+      ).toThrow(/needs.*relatedDocument/);
+    });
+  });
+
   it("throws when the lines do not sum to the invoice total", () => {
     expect(() => toEimsInvoice(invoice({ totalAmount: "9000.00" }), seller, context())).toThrow(
       /lines sum to 11000 but the invoice total is 9000/,
diff --git a/apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.ts b/apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.ts
index fb7f2f145..582e3d326 100644
--- a/apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.ts
+++ b/apps/edr-freight-api/src/modules/billing/eims-invoice.mapper.ts
@@ -20,8 +20,15 @@ import { round2 } from "./invoice-settlement.util";
 /** Only proven-required constant: the 400 SCHEMA ERROR sample rejects a payload without it. */
 const EIMS_VERSION = "1";
 
-/** The only `DocumentDetails.Type` observed in the supplied material. */
-const EIMS_DOCUMENT_TYPE = "INV";
+/**
+ * `DocumentDetails.Type`. `"INV"` is the only value observed in the collection; `"DEB"`/`"CRE"`
+ * (debit/credit note) were confirmed directly by MoR support — same `/v1/register` endpoint, no
+ * separate API. MoR's answer, verbatim: "the same endpoint used for registration should be used
+ * ... within the Document Detail object, you should specify DEB for a debit note, CRE for a
+ * credit note... add a Reason attribute under document detail object".
+ */
+export const EIMS_DOCUMENT_TYPES = ["INV", "DEB", "CRE"] as const;
+export type EimsDocumentType = (typeof EIMS_DOCUMENT_TYPES)[number];
 
 export interface EimsBuyerDetails {
   City: string | null;
@@ -60,7 +67,9 @@ export interface EimsDocumentDetails {
   DocumentNumber: string;
   /** Observed format `dd-MM-yyyyTHH:mm:ss`. Rule seen in the collection: within 3 days of now. */
   Date: string;
-  Type: string;
+  Type: EimsDocumentType;
+  /** Only for DEB/CRE, per MoR support — why the debit/credit note was issued. Absent for INV. */
+  Reason?: string;
 }
 
 export interface EimsInvoiceItem {
@@ -212,7 +221,18 @@ export interface EimsMapperContext {
   unitDefault: string;
   incomeWithholdValue: number;
   transactionWithholdValue: number;
-  /** Null for an ordinary invoice; set only for a real related-document case. */
+  /**
+   * `DocumentDetails.Type`. Defaults to `"INV"`. For `"DEB"`/`"CRE"` both `reason` and
+   * `relatedDocument` become required — confirmed directly by MoR support, not the collection.
+   */
+  documentType?: EimsDocumentType;
+  /** Required when `documentType` is `"DEB"`/`"CRE"` — why the note was issued. Unused for INV. */
+  reason?: string | null;
+  /**
+   * `ReferenceDetails.RelatedDocument`. Null for an ordinary invoice; required for a DEB/CRE —
+   * the original registered invoice's IRN, per MoR's own IRC-P06/P07 checklist ("credit memo
+   * from a registered invoice").
+   */
   relatedDocument?: string | null;
   /** MoR numeric country code for the buyer; our DB stores the country name. */
   buyerCountryCode?: string | null;
@@ -326,6 +346,26 @@ export function toEimsInvoice(
     );
   }
 
+  const documentType = context.documentType ?? "INV";
+  if (!EIMS_DOCUMENT_TYPES.includes(documentType)) {
+    throw new Error(
+      `EIMS mapping: invoice ${invoice.invoiceNumber} has documentType "${documentType}", must be one of ${EIMS_DOCUMENT_TYPES.join(", ")}`,
+    );
+  }
+  if (documentType !== "INV") {
+    if (!context.reason?.trim()) {
+      throw new Error(
+        `EIMS mapping: invoice ${invoice.invoiceNumber} is a ${documentType} (debit/credit note) and needs a reason`,
+      );
+    }
+    if (!context.relatedDocument?.trim()) {
+      throw new Error(
+        `EIMS mapping: invoice ${invoice.invoiceNumber} is a ${documentType} (debit/credit note) and needs ` +
+          "relatedDocument — the original registered invoice's IRN",
+      );
+    }
+  }
+
   const issuedAt = invoice.issuedAt instanceof Date ? invoice.issuedAt : new Date(invoice.issuedAt);
   if (Number.isNaN(issuedAt.getTime())) {
     throw new Error(`EIMS mapping: invoice ${invoice.invoiceNumber} has an unparseable issuedAt`);
@@ -430,7 +470,8 @@ export function toEimsInvoice(
     DocumentDetails: {
       DocumentNumber: context.documentNumber,
       Date: (context.formatDate ?? formatEimsDate)(issuedAt),
-      Type: EIMS_DOCUMENT_TYPE,
+      Type: documentType,
+      ...(documentType !== "INV" ? { Reason: context.reason! } : {}),
     },
     ItemList,
     PaymentDetails: { Mode: context.payment.mode, PaymentTerm: context.payment.term },
diff --git a/apps/edr-freight-api/src/modules/billing/entities/invoice.entity.ts b/apps/edr-freight-api/src/modules/billing/entities/invoice.entity.ts
index 7d0ff34c4..43b6e3271 100644
--- a/apps/edr-freight-api/src/modules/billing/entities/invoice.entity.ts
+++ b/apps/edr-freight-api/src/modules/billing/entities/invoice.entity.ts
@@ -180,4 +180,27 @@ export class Invoice extends BaseEntity {
 
   @Column({ name: "eims_cancellation_remark", type: "text", nullable: true })
   eimsCancellationRemark?: string | null;
+
+  /**
+   * `DocumentDetails.Type` to file this invoice as — "INV" (default), "DEB" or "CRE". Confirmed
+   * by MoR support directly (not the collection): debit/credit notes go through this same
+   * `/v1/register` endpoint, distinguished only by `Type` + `Reason`, linked via
+   * `ReferenceDetails.RelatedDocument` to the original invoice's IRN. This module does not create
+   * debit/credit note invoices — that is a freight-workflow decision — it only files one
+   * correctly once these columns are set on an existing row.
+   */
+  @Column({ name: "eims_document_type", type: "varchar", length: 8, default: "INV" })
+  eimsDocumentType!: string;
+
+  /** Required by MoR when `eimsDocumentType` is DEB/CRE — why the note was issued. */
+  @Column({ name: "eims_reason", type: "text", nullable: true })
+  eimsReason?: string | null;
+
+  /** The original registered invoice this debit/credit note adjusts. Required for DEB/CRE. */
+  @Column({ name: "related_invoice_id", type: "uuid", nullable: true })
+  relatedInvoiceId?: string | null;
+
+  @ManyToOne(() => Invoice)
+  @JoinColumn({ name: "related_invoice_id" })
+  relatedInvoice?: Invoice | null;
 }
diff --git a/apps/edr-freight-api/src/modules/eims/eims-invoice-context.ts b/apps/edr-freight-api/src/modules/eims/eims-invoice-context.ts
index 8e7be0197..b77804eda 100644
--- a/apps/edr-freight-api/src/modules/eims/eims-invoice-context.ts
+++ b/apps/edr-freight-api/src/modules/eims/eims-invoice-context.ts
@@ -157,6 +157,12 @@ export interface EimsContextInput {
   session: EimsSessionContext;
   /** Required when the invoice currency is not ETB. */
   exchangeRate?: number | null;
+  /** `DocumentDetails.Type` — defaults to "INV" in the mapper when omitted. */
+  documentType?: EimsMapperContext["documentType"];
+  /** Required (by the mapper) when documentType is DEB/CRE. */
+  reason?: string | null;
+  /** `ReferenceDetails.RelatedDocument` — the original invoice's IRN, required for DEB/CRE. */
+  relatedDocument?: string | null;
 }
 
 export function buildEimsContext(config: EimsConfig, input: EimsContextInput): EimsMapperContext {
@@ -206,5 +212,8 @@ export function buildEimsContext(config: EimsConfig, input: EimsContextInput): E
     buyerIdType: invoice.buyerIdType,
     buyerIdNumber: invoice.buyerIdNumber,
     exchangeRate: input.exchangeRate ?? null,
+    documentType: input.documentType,
+    reason: input.reason ?? null,
+    relatedDocument: input.relatedDocument ?? null,
   };
 }
diff --git a/apps/edr-freight-api/src/modules/eims/eims-invoice-registration.service.spec.ts b/apps/edr-freight-api/src/modules/eims/eims-invoice-registration.service.spec.ts
index 8c2a33cfc..36c355bed 100644
--- a/apps/edr-freight-api/src/modules/eims/eims-invoice-registration.service.spec.ts
+++ b/apps/edr-freight-api/src/modules/eims/eims-invoice-registration.service.spec.ts
@@ -295,6 +295,58 @@ describe("EimsInvoiceRegistrationService.registerInvoiceWithEims", () => {
     expect(request.SourceSystem.SystemNumber).toBe(SYSTEM_NUMBER);
   });
 
+  it("files a credit note with Type/Reason/RelatedDocument from the invoice row", async () => {
+    const original = invoiceRow({
+      id: "original-invoice",
+      invoiceNumber: "INV-20260807-00001",
+      eimsIrn: IRN,
+    });
+    const db = new FakeDb([
+      invoiceRow({
+        eimsDocumentType: "CRE",
+        eimsReason: "Overbilled freight charge",
+        relatedInvoice: original,
+      } as Partial<Invoice>),
+    ]);
+    const postSigned = jest.fn().mockResolvedValue(okResponse());
+
+    await build(db, postSigned).registerInvoiceWithEims(INVOICE_ID);
+
+    const request = postSigned.mock.calls[0][1] as EimsInvoiceRequest;
+    expect(request.DocumentDetails).toMatchObject({ Type: "CRE", Reason: "Overbilled freight charge" });
+    expect(request.ReferenceDetails.RelatedDocument).toBe(IRN);
+  });
+
+  it("refuses a credit/debit note whose related invoice was never registered, before touching a counter", async () => {
+    const original = invoiceRow({ id: "original-invoice", eimsIrn: null });
+    const db = new FakeDb([
+      invoiceRow({
+        eimsDocumentType: "DEB",
+        eimsReason: "Additional handling",
+        relatedInvoice: original,
+      } as Partial<Invoice>),
+    ]);
+    const postSigned = jest.fn();
+
+    await expect(build(db, postSigned).registerInvoiceWithEims(INVOICE_ID)).rejects.toBeInstanceOf(
+      BadRequestException,
+    );
+    expect(postSigned).not.toHaveBeenCalled();
+    expect(db.state).toMatchObject({ nextInvoiceCounter: 7 }); // unchanged — never reserved
+  });
+
+  it("refuses a credit/debit note with no related invoice set at all", async () => {
+    const db = new FakeDb([
+      invoiceRow({ eimsDocumentType: "CRE", eimsReason: "x", relatedInvoice: null } as Partial<Invoice>),
+    ]);
+    const postSigned = jest.fn();
+
+    await expect(build(db, postSigned).registerInvoiceWithEims(INVOICE_ID)).rejects.toBeInstanceOf(
+      BadRequestException,
+    );
+    expect(postSigned).not.toHaveBeenCalled();
+  });
+
   it("takes SourceSystem from the token session, not from configuration", async () => {
     const db = new FakeDb([invoiceRow()]);
     const postSigned = jest.fn().mockResolvedValue(okResponse());
diff --git a/apps/edr-freight-api/src/modules/eims/eims-invoice-registration.service.ts b/apps/edr-freight-api/src/modules/eims/eims-invoice-registration.service.ts
index 317f4ec65..ab9f953c5 100644
--- a/apps/edr-freight-api/src/modules/eims/eims-invoice-registration.service.ts
+++ b/apps/edr-freight-api/src/modules/eims/eims-invoice-registration.service.ts
@@ -13,6 +13,7 @@ import type { QueryDeepPartialEntity } from "typeorm/query-builder/QueryPartialE
 import { EimsConfig } from "../../config/eims.config";
 import { Invoice } from "../billing/entities/invoice.entity";
 import {
+  EimsDocumentType,
   EimsInvoiceRequest,
   EimsMapperLine,
   toEimsInvoice,
@@ -96,6 +97,27 @@ export class EimsInvoiceRegistrationService {
     const invoice = await this.loadInvoiceForMapping(invoiceId);
     if (invoice.eimsIrn) return this.toView(invoice);
 
+    // Debit/credit notes (confirmed by MoR support: same endpoint, Type DEB/CRE + Reason,
+    // ReferenceDetails.RelatedDocument = the original's IRN) must fail here — before a counter is
+    // touched — if the original was never actually registered.
+    const documentType = (invoice.eimsDocumentType as EimsDocumentType | undefined) ?? "INV";
+    let relatedDocument: string | null = null;
+    if (documentType !== "INV") {
+      if (!invoice.relatedInvoice) {
+        throw new BadRequestException({
+          code: "EIMS_RELATED_INVOICE_REQUIRED",
+          message: `Invoice ${invoice.invoiceNumber} is a ${documentType} but has no related invoice set.`,
+        });
+      }
+      if (!invoice.relatedInvoice.eimsIrn) {
+        throw new BadRequestException({
+          code: "EIMS_RELATED_INVOICE_NOT_REGISTERED",
+          message: `Invoice ${invoice.invoiceNumber} is a ${documentType} against invoice ${invoice.relatedInvoice.invoiceNumber}, which was never registered with EIMS — nothing to reference.`,
+        });
+      }
+      relatedDocument = invoice.relatedInvoice.eimsIrn;
+    }
+
     // Authenticate before reserving: the source system comes from the token, and the state row is
     // keyed by it. A login failure here costs nothing — no counter has been consumed yet.
     const session = await this.auth.getSessionContext();
@@ -114,6 +136,9 @@ export class EimsInvoiceRegistrationService {
         invoiceCounter: reservation.invoiceCounter,
         previousIrn: reservation.previousIrn,
         session,
+        documentType,
+        reason: invoice.eimsReason,
+        relatedDocument,
       }),
     );
 
@@ -639,7 +664,7 @@ export class EimsInvoiceRegistrationService {
   ): Promise<Invoice & { lines: EimsMapperLine[] }> {
     const invoice = await this.dataSource.getRepository(Invoice).findOne({
       where: { id: invoiceId },
-      relations: { company: true, companyProfile: true },
+      relations: { company: true, companyProfile: true, relatedInvoice: true },
     });
     if (!invoice) throw new NotFoundException(`Invoice ${invoiceId} not found`);