Refund of overpayment versus carrying a credit on the account.

I have a resident who overpaid this rent. He got a money order for 768.00 to pay a 7.68 gas bill. He submitted the payemnt to our office. We called him and he did not respond to the message so we put the money in the bank. This resident is now wanting the money back. This is a large management company and to do a refund will take approx 30 days. At this point the next rental period will be upon us and the credit will be almost used up. Are we legally bound to refund this persons money. The credit is in his account.