Is failing to return the excess amount dispensed by the teller considered a sin, and how can one atone for that?

If you receive an excess amount of money from a bank or ATM that was not deducted from your account, you must return this amount to its rightful owner, whether it is the bank or the employee, if they are the one who will bear the mistake. Repentance is not valid unless the money is returned, and there is remorse for not returning it, and a firm resolve not to repeat such an act. It must be noted that it is not permissible to deposit money in usurious banks except out of extreme necessity and only in a current account.