Every modern economy ships a real-time payment system in the 2010s-2020s: NIBSS Instant Pay (Nigeria 2011), IMPS/UPI (India 2010/2016), Pix (Brazil 2020), FedNow (US 2023), RTP (US 2017), FAST (Singapore), SEPA Instant (EU). Different jurisdictions, identical engineering shape: 24/7, sub-minute settlement, irrevocable on receipt. The fintech's ledger gets a credit on arrival and immediately marks the funds cleared. The interesting accounting happens at the BOUNDARY: what to do when a customer sends to a wrong recipient, when settlement fails mid-flight, and how interoperability fees flow.