You would turn the money in at the member service desk or to a store manager! The interviewer needs to hear that you are honest and ethical. In this situation, the best thing to do is to turn the money in, and it is what the interviewer expects to hear from you.
"I could never keep money that was not mine. Perhaps the person who dropped the money may desperately need that $20 today, and turning it in would give them a better chance of finding the money."