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What is bait money?
Short answer
Bait money is cash a bank keeps in teller drawers with every serial number written down. If it's handed over in a robbery, those bills can be traced when they turn up.
You can't tell bait bills from normal ones by looking at them. That's the point.
It's why rule one in Part 1 is never to spend it near where you took it.
Sources: Common bank security practice
US federal rules, explained for a story. Not legal advice.