פירוש על קידושין 55:9

The general rule of acquiring things through exchange is illustrated simply in the example of the cow and ox. If Reuven and Shimon exchange an ox for a cow, when Reuven takes physical possession of Shimon’s cow, Shimon becomes owner of the ox, even if Shimon doesn’t take physical possession. The implication would be that if the ox dies or is stolen, Shimon is out of luck for it is his ox that died or was stolen. Alternatively, if the oxen market rises dramatically Shimon wins out. For better or for worse, in an exchange once one party takes possession of one of the objects being exchanged, the other party automatically owns the other object.