Rav Yehuda quotes Shmuel as offering the following ruling:
With regard to one who exchanges a cow for a donkey, where the two animals involved in this transaction are not in the same location, one of the parties acquires one of the animals by means of pulling it, which transfers the other animal to the other party through acquisition by means of the exchange. And in this case the owner of the donkey pulled the cow, but before the owner of the cow could pull the donkey in turn, the donkey died. The owner of the cow claimed that the donkey died before the other one pulled the cow, which means the exchange transaction never took effect. In that case,the owner of the donkey must bring proof that his donkey was alive at the time when the cow was pulled. If he is unable to bring proof to this effect, the owner of the cow retains his animal.
Many rishonim point out that this ruling is fraught with difficulties, as there are two pressing reasons to rule that the owner of the donkey should be in an advantageous position in this case:
- He is already in possession of the cow.
- We know that the donkey was alive when the negotiations began, and we have no reason to assume that it died before the transaction took place.
Tosafot raise these issues and offer two approaches. First of all, the owner of the cow has hezkat mara kamma – he was the definitive, original owner – and it is not clear to us that the owner of the donkey successfully performed an act that removed it from his possession, so the burden of proof is on the owner of the donkey. Secondly, we need to evaluate the situation based on the facts that stand before us now. We know for certain that the donkey is dead. In order for us to be convinced that the transfer took place while the donkey was still alive, the owner of the donkey needs to show that the situation that exists now (i.e. the donkey is dead) was different at that time.
In his Sefer haYashar, Rabbeinu Tam suggests that the ruling here is based on the fact that the donkey was found dead while still in the confines of its original owner’s farm, which is why we demand information from him. According to this approach, had the animal been moved to a public area, Shmuel’s ruling would not apply.
One other explanation is offered by the Ra’avad and Re’ah, who argue that in this case – given that the donkey is dead – we cannot be certain that the exchange took place at all, so we leave everything as it was before the exchange took place, unless the donkey owner can prove that the change of ownership successfully happened.