audit-payment-system — Money-Movement Correctness & Compliance Audit

Payment code fails differently from normal CRUD. A dropped PUT is an annoyance; a retried charge is a double-charge, a lost ledger write is money that vanished, a logged PAN is PCI liability, and an unverified webhook is a spoofed "payment succeeded". The bugs are silent — everything looks green until a customer is charged twice or the month-end books don't balance. This audit checks the controls that keep every cent accounted for and every charge exactly-once.

The 2026 consensus is consistent across sources: a payment system rests on three pillars — idempotency, a double-entry ledger, and reconciliation — with PCI DSS v4.0.1 as a non-negotiable floor and webhooks as the source of truth (never trust the synchronous API response alone).