Have you ever walked out of a wedding or out of the synagogue, gone to the cloakroom to get your coat only to discover that someone had taken yours and left a similar one in its place? While you hope that yours will be returned (which is why you should leave a siman – like your name and address in it), in the meanwhile can you use the other person’s coat?
The Gemara on today’s daf quotes a baraita that deals with this very issue. According to the baraita, if a person gives something to a tailor or repairman to be fixed and receives the wrong item in return, he can use it until the person who got his belonging by accident returns it and takes his own item. If, however, he finds that his coat was switched at a party or at the house of a mourner (i.e. places where large crowds gather and such mistakes happen), he cannot use it until he locates the true owner and returns the coat, receiving his own in exchange.
In explaining the difference between these two cases, Rav says that Rabbi Ḥiyya explained to him that ordinarily we cannot use other people’s property without their permission. In the case where the wrong item was given to him by the repairman, it is at least possible that the item he received had been given to the repairman to fix and then sell. Therefore he is receiving an object that the repairman had the right to give him.
Rav Ḥiyya the son of Rav Naḥman limits this ruling only to situations where the repairman himself gave him the object, because only there can we apply Rabbi Ḥiyya’s reasoning. If the repairman’s wife or child gave him the object, then it is likely that it was simply a mistake, and he has no permission to use the object.