This is the cashier's mistake, and the consequences should be borne by the cashier. According to reason, you should sign for confirmation first, and then pay cash. Otherwise, you gave cash and the other party didn't sign for confirmation. When you came back, you asked for it. You don't have any proof that he took your cash, unless there is a surveillance camera or something. Even if there is such evidence, it is your dereliction of duty. If the other person is easy to communicate and considerate, I'll come back and sign for you. Just remember, sign first, then pay.