The summary.
BUILD AROUND AN IMMUTABLE LEDGER AS THE SOURCE OF TRUTH
Every balance, statement and report derives from it. Enforce every money movement through one path — several paths diverge and reconciliation stops being trustworthy.
CHECK-THEN-ACT FAILS UNDER CONCURRENCY
Two requests both read a sufficient balance before either writes. Lock, constrain against negative balances, and test concurrent operations deliberately — these defects never appear in sequential testing.
PROTECT THE AUDIT TRAIL SEPARATELY
Anyone able to alter records can alter the trail. And test reconstructing a past transaction entirely from records, because that is exactly what a dispute demands.
THE FIRST QUESTION AFTER A FAILURE IS ALWAYS WHERE THE MONEY IS
Answer it in the interface. Never make an agent guess whether a transaction succeeded.
ESTABLISH THE REGULATED ACTIVITY AND LICENSING ROUTE BEFORE BUILDING
Partnership terms will constrain your product, so negotiate settlement, data access and exit provisions carefully — your customers and balances sit in their infrastructure.
Launch with deliberately low limits; they contain the damage from whatever you missed.
COST TO SERVE SCALES WITH CUSTOMERS, NOT REVENUE
Support, compliance and failed-transaction handling are what make unit economics fail.
