Tosphot asks in the beginning of Bava Batra why do you need "therefore" [that is since they are required to build the wall, therefore they divide if it falls.]. Answers Tosfot: it might fall into the domain of just one and he would be believed saying, "I built it" because he has a migo [i.e a case of "he could have said such and such and be believed, so if he puts in a different plea, he should be believed. For after all if he wanted to lie, he had a better way of doing so."] of saying, "I bought it."
Rav Akiva Eigger asks, "Why do you need 'I bought it?'" Perhaps just "I built it" alone should be believed since it is in his domain except for the "therefore" of the mishna.
Rav Shach says if he would say "I built it", and if he is believed, that takes the wall out of its hazaka [prior status] of belonging to both. [He got the idea from Isar Meltzer the author of the Even HaEzel]
So to say, "I built it" would be believed only because "I bought it" would be believed. But as the mishna says here neither would be believed because both are required to build the wall.
Rav Shach suggests further that this might depend on a similar argument between Tosphot and the Rambam in Bava Metzia 6b. The case is two people come into court holding a garment. The law is they divide. What happens if after that, one comes in and only he is holding it and says, "The other admitted to me that it is mine."? The other says, "I rented it to him." The Gemara says, "He is not believed, because we say 'Until now you thought he is a thief, and now you rented it to him without witnesses?"
Tosfot asks why do we need the "We are witnesses?" Answer: because there is a migo he could have said, "You grabbed it from me." So we see that in fact if he had said that he would be believed. So why not believe the first one that has the object? Because he says you agreed with me placing the other in the category of a thief and so he is not believed.
It is times like this that I wish I was learning with David Bronson, my learning partner in Uman. For what Rav Shach is saying here is hard to figure out on my own. It seems to me that both are accusing the other. And why would this have a Hezkat [prior status] movable objects after they were already in court and it was decided they should split.
I would like to suggest that this is in fact the reason the Gra [in Choshen Mishpat 138] and the Ritva have a different answer for why he would be believed to say, "The other grabbed it," because it is talking about things that are commonly borrowed or rented out.
[That is what I think that note of the Gra means over there.]
Furthermore the Rambam is consistent with his other opinion about if one grabs after there is already a doubt that we do not take it from him and the Rosh holds we do take it from him. [That is about the case of "a cohen grabs a animal of tithe that is doubtful."]
So the Rosh is just going with the Tosphot as usual. That is to say both of the pleas cancel since each is accusing the other of lying so we simply go with hazaka. And the one that has it now has no hazaka since he has it after there has already been born the doubt.
However the reason why I think the Gra is right here is that Tosphot is saying that the plea "he took it from me" [in the Bava Metzia case on page 6] is believed in and of itself, not just because of the hezkat metatalim [status of movable objects.] So while the issue of one party seized it after the doubt is born is relevant still that is not the reason for Tosphot to say the actual plea of he seized it is believed.
