A family ordered eight platters of vegetable sushi for a kiddush. On Shabbos morning, they discovered that two of the platters were made with fish, which costs significantly more.
Q: May they eat the sushi and only pay the cost of vegetable sushi, or should they return it after Shabbos, at which time it will be worthless to the caterer?
A: Obviously, if it were possible to correct the error by returning the wrong platters for the correct ones, that would be preferable. In this circumstance, that is not realistic. However, even if the caterer will have no use for the returned trays, it should still not be permitted for the family to use those trays based on the principle of yi’ush shelo mi’daas abandonment without the owner’s knowledge. This refers to a case where an owner loses an object that he would abandon hope of recovering, but he has yet to realize that it is lost. The Gemara’s conclusion is that it is not yet considered abandoned (Bava Metzia 21b-22b). Accordingly, since the caterer may not be aware that he sent fish sushi rather than vegetable sushi, it should be prohibited for the family to serve those platters based on the principle of yi’ush shelo mi’daas.
Despite these arguments, the halacha is that it is permitted for them to serve the fish. They will only be obligated to pay the value of a vegetable sushi platter. It can be assumed that the caterer would not want the family to just put aside the fish sushi trays, since this would result in the caterer suffering a complete loss from those trays. Moreover, if the family does not serve the two additional trays, it will damage the caterer’s reputation, since it will seem that he is not reliable in preparing an order. Therefore, one has the right to assume that the caterer is interested in maintaining the original agreement, providing eight platters of food at the cheaper rate, because by doing so he will at least recover most of the cost of the fish sushi. Thus, since both parties have a desire for the original agreement to go forward, it is permitted for the family to serve that sushi and they will only be responsible to pay the price of the cheaper trays.