That is pretty much how it’s done today. Global transactions are authorised in real time and then settled periodically. If I remember correctly, Visa does a settlement every 3 or 4 hours, then each of the merchants and other processors in the chain run their own schedule, so it can take a few days for balances to correctly reflect a transaction.

I remember being very impressed by Spanner when I first heard about it, but as you can see in the OP, it does make a lot of consistency guarantees through sheer brute force. It simply throws power/resources at problems which would generally be considered impractical when normally designing a distributed system. An approach which does of course have its limits (not to say it isn’t still impressive, it’s a great system).