So A needs some kind of global ledger to keep track of how much cash B actually has, and double check whether it's already told C, D, E that they have all of it

So A needs some kind of global ledger to keep track of how much cash B actually has, and double check whether it's already told C, D, E that they have all of it. https://en.m.wikipedia.org/wiki/Nostro_and_vostro_accounts https://en.m.wikipedia.org/wiki/Nostro_and_vostro_accounts This is a long solved problem. No blockchains necessary. As an aside this is a problem I often see in fintech pitches...