No matter how you model things and no matter what technology you use, a race condition like this needs to be handled one way or another. You either handle it su

No matter how you model things and no matter what technology you use, a race condition like this needs to be handled one way or another. You either handle it such that the data is always consistent, or you handle inconsistent data. You can use an SQL database with transaction and locking to ensure that you will never debit money that isn't there. Or you can save commands in a queue that only a single process at any given time (that incidentally includes the SQL scenario). Or you can use a distributed consensus algorithm with multiple stage commits. There is no way around it.