Overpayment – Keep the change
Sender asks Chaim to pick up a bottle of detergent at the local supermarket. The bottle costs $9.50 and Chaim lays out the money.
Sender repays Chaim with a $10 bill and tells him to keep the change. Has Sender payed ribbis?
Does it make any difference if Sender says he is giving the extra money as compensation for the effort of shlepping the detergent from the store?
According to many poskim this is a form of ribbis, and therefore it would have been better for Sender to give Chaim his own money beforehand to buy it on his behalf.
Since this was not the case, when Chaim lays out his own money it is as if he is lending that money to Sender, who has now repaid more than he borrowed.
Many have tried avoiding this issue (and even larger ribbis issues) by claiming that the money “was given as a present.” This is not a valid heter (though some poskim are lenient when done between relatives where it is obvious that it was done as a present; see footnote).
The poskim offer various solutions to avoid our problem.
One way would be for Chaim to buy the detergent for himself, and then resell it to Sender for ten dollars
Another option would be for Sender to say that he is giving the extra money as reimbursement for the effort Chaim expended on his behalf.