Anytime a customer receives cash, they should sign for it. I can't imagine why anyone wouldn't? How do you ever show that cash back was received? Also, it's for the teller's protection. Say someone disputes the transaction, without a signature how do you know the customer got it? If I put myself in your tellers shoes, I would want that verification. If I put myself in a fraudster-teller shoes, I'd want to work for your bank.
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"The reason I talk to myself is because I'm the only one whose answers I accept."
- George Carlin