Q: I found a book with identifying features. While hanging a notice, I heard someone telling a friend that he lost his book and now abandoned hope of finding it. Must I still return it?
A: When you find an item and pick it up before the owner’s yei’ush, you are required to return it even if the owner subsequently had yei’ush (C.M. 259:1; 262:3).
Tosfos (B.K. 66a) explains that once the item comes to your hands before yei’ush — because you are required to return it, the obligation does not cease even after yei’ush, similar to a thief who is obligated to return the theft and the obligation does not cease upon yei’ush of the owner (C.M. 353:2; Drush v’Chiddush B.M. 21b; Chazon Ish B.K. 18:1).
Ramban (Milchamos B.M. 26b) explains that once you take the item to return it, you become a guardian on behalf of the owner, and, as we previously learned, yei’ush does not take force when the item is secure in the hands of the owner or his guardian, and the owner’s yei’ush is in error (Ketzos 259:1).
