Not the OP but I think it's essentially a event driven system as opposed to real time.

Think of a account with $0 in it.

1. A -$25 event comes in

2. A +$50 event comes in

3. A -$25 event comes in

The first event would fail in a transactional/real time system, but in my bank it doesn't. This tells me the balance is only calculated every once in a while. The balance you get in a app is only a estimate.

This makes sense when you think how slow bank transfers and using paper checks is. At some point it is decided to run the events "for real" and that's where overdrafts and the like are applied.

Don't think of them as transactions, instead it's a transaction request that can be rejected.