A non-Jewish lawyer gave a Jewish lawyer a retainer of $350 to be an expert witness on a case. The case subsequently closed without the Jew having to be a witness at all. The non-Jewish lawyer has not requested his retainer back, presumably because he forgot. Does the Jewish lawyer have to return the money? Is he allowed to return the money?
אולי נחשב הקפעת הלוותו ואין צריך להחזיר המעות, אך אולי נחשב גם כהשבת אבידה ולפ”ז אסור להחזיר אם לא היכא שיש קידוש השם
The money may certainly be returned. This is not considered a loan rather payment for a service which was never delivered. This still falls under the category of טעות עכו”ם and as such there is no חיוב to return the money. However being that it would be a kiddush Hashem this would be a commendable practice.