What is the ruling on a seller doubting that the remainder of the price was not fully returned, and is the money returned by the seller to the buyer lawful for the buyer, given that the buyer does not remember?

If the seller has received the money, his liability is definitively established and is not discharged except by returning the remaining amount to the buyer. As for the buyer, he is not permitted to take the money again if he knows he previously received it; otherwise, he is permitted to do so.