You missed an important bit... the banks (A & B) accounts at the federal reserve are updated (bank A down, bank B up) for the transfer. And that's where the rubber meets the road. If bank A doesn't have the assets, it all stops. Banks don't just give each other endless credit to solve payments...
There is no magic in banking. If you describe something and it sounds magical, a piece is missing. If you were running bank B, you'd never agree to what you described. You'd want the assets, or you'd want some kind of collateral even if you were willing to do an interbank credit, you'd limit it, you'd do all kinds of credit analysis on your bank counterparties... And what I just described is how trading of securities tends to work between banks. But even then, it's not how payments are solved...
There is no magic in banking. If you describe something and it sounds magical, a piece is missing. If you were running bank B, you'd never agree to what you described. You'd want the assets, or you'd want some kind of collateral even if you were willing to do an interbank credit, you'd limit it, you'd do all kinds of credit analysis on your bank counterparties... And what I just described is how trading of securities tends to work between banks. But even then, it's not how payments are solved...