When I say message I’m think of the equivalent of a TCP packet. So you have a bunch of error checking and ACKs flying about. But only one message to actually move money.

Single message systems already exist, most US card transactions move over a single message system.

> there can’t be an option for mistakes!

This is where you’re wrong. Mistakes happen all the time when moving money, a big part of any banks operations is cleaning up those mistakes.