Q: A visitor left a package of cookies in our dorm room and declared them hekfer (ownerless). My roommate took them for himself, while I claim that we already acquired them jointly. Whose are they?
A: The Gemara (B.B. 84b) teaches that partners can acquire through kinyan chatzer in a joint courtyard. Based on this, the Rosh and Rema rule that joint property acquires hefker on behalf of both partners (C.M. 260:4).
The Ketzos Hashulchan disagrees with this ruling, and maintains that joint property does not acquire hefker for both unless they are partners in all financial matters, so that whoever subsequently takes the hefker acquires it. However, later Acharonim uphold the Rema’s ruling, so that the cookies should be shared (Ketzos 260:1; Aruch Hashulchan 260:9; Tabaas Hachoshen 260).
Nonetheless, if the roommate intended from the beginning to take the cookies for himself and did not want the property to acquire jointly, the Nesivos concurs that he alone acquires them (Nesivos 260:9).