reject cents mismatch on CBE bills

apps/edr-payment-api/src/modules/cbe-bill/cbe-bill.amount.spec.ts

@@ -0,0 +1,22 @@
+import { amountsMatchToTheCent } from "./cbe-bill.service";
+
+describe("amountsMatchToTheCent (CBE payment amount gate)", () => {
+  it("accepts the exact amount", () => {
+    expect(amountsMatchToTheCent(1234.34, 1234.34)).toBe(true);
+  });
+
+  it("rejects a cents-only difference (the 1234.89 vs 1234.34 bug)", () => {
+    expect(amountsMatchToTheCent(1234.89, 1234.34)).toBe(false);
+    expect(amountsMatchToTheCent(1234.35, 1234.34)).toBe(false);
+  });
+
+  it("rejects whole-unit differences", () => {
+    expect(amountsMatchToTheCent(1235.34, 1234.34)).toBe(false);
+  });
+
+  it("absorbs double-precision storage noise", () => {
+    expect(amountsMatchToTheCent(1234.34, 1234.3399999999999)).toBe(true);
+    // classic float artifact: 0.1 + 0.2 !== 0.3
+    expect(amountsMatchToTheCent(0.1 + 0.2, 0.3)).toBe(true);
+  });
+});

apps/edr-payment-api/src/modules/cbe-bill/cbe-bill.service.ts

@@ -38,8 +38,15 @@ import {
 /** Postgres unique_violation — the DB-level idempotency backstop firing on a concurrent duplicate. */
 const PG_UNIQUE_VIOLATION = "23505";
 
-/** Mirrors the short-pay tolerance already applied in handlePaymentEvent. */
-const AMOUNT_TOLERANCE = 0.01;
+/**
+ * CBE must pay the bill to the exact cent — compare in integer cents so
+ * double-precision storage noise (1234.34 stored as 1234.33999…) can neither
+ * mask nor fabricate a difference. A relative tolerance is wrong here: 1% of a
+ * 1234.34 bill would wave through anything up to ±12.34.
+ */
+export function amountsMatchToTheCent(a: number, b: number): boolean {
+  return Math.round(a * 100) === Math.round(b * 100);
+}
 
 /**
  * Translate an intent's own terminal state into the same reason vocabulary the domain apps
@@ -268,8 +275,7 @@ export class CbeBillService {
       const amount = Number(dto.Amount);
       if (
         !Number.isFinite(amount) ||
-        Math.abs(amount - intent.amountMinor) >
-          intent.amountMinor * AMOUNT_TOLERANCE
+        !amountsMatchToTheCent(amount, intent.amountMinor)
       ) {
         throw new CbeBillError("Amount mismatch", "BUSINESS");
       }