I've probably gotten off base here by wanting to perform arbitrary actions against services I may or may not control in the course of satisfying a command, and

I've probably gotten off base here by wanting to perform arbitrary actions against services I may or may not control in the course of satisfying a command, and worrying too much about made up corner cases. If this DB is the sole record of The Money, and I can move some quantity from X to Y in a transaction, then that's fine by me.