If a client overpays, those additional funds belong to the client and at no point should the law firm keep them. It is the lawyer’s responsibility to make sure those funds make it back to the client.
Those funds should be placed back in the client trust account and a diligent effort made to reimburse them for the overage. The easiest way to handle the matter is to write a check for the additional funds and send it to the client at their last known address. If the check comes back to the firm, you should continue to attempt to locate a new address for the recipient.
Keep in mind that there is no amount too small to be placed into the trust account. Even if the invoice was for $5,400 and the client paid $5,425, the $25 should still be placed in the trust account. Those funds don’t belong to the firm and therefore should not remain in the operating account. Doing so puts the firm at risk of compliance issues regarding the commingling of funds.