This is bunk. The cashier has no way to hit cash back on the register keyboard. It is on the customer’s keypad on the debit reader. Alot of confused customers hit the wrong button and then want to blame the cashier. He or she has no control over that. My guess is that the e-mail is from a dissatisfied person who hit cash back instead of no cash back.
It’s possible that the quick thinking cashier noticed that the shopper hit the cash back button in error and decided to keep the cash.