"Wait, you have some change coming," said my cashier.
"Oh, that's OK, you can keep it," he responded, and quickly made his way out of the front door, incidentally setting off the metal detector (or whatever you call the thing that's supposed to detect when you've got items that haven't been paid for).
The security guard looked on, a bit uncertain what to do. After a few seconds, the cashier told him, "Go and bring him back here!" The guard left the Brooks and returned a short while later with the guy in question, who went through the metal detector again without setting anything off and was permitted to leave.
The cashier then explained to the guard that part of being a security guard is that if it seems like someone might be stealing stuff from the store then you have to try to stop them. (I think it was his first day on the job.)