Bava Batra

Why should this be, seeing that this is a case of keeping with the owner present?  — R. Papa replied: [Samuel's rule applies] where one said to the other, You keep [the whole property for me] today and I will keep it [for you] tomorrow. Our Rabbis taught: If a man sells to another a house or a field, he is not allowed to testify to the latter's title to it  because he is responsible to him for it.  If, however, he sells him a cow or a garment, he can testify to his title to it, because he is not responsible to him for it. Why should the rule in the second case be different from that in the first? — R. Shesheth said: The first rule [applies to a case where, for instance,] Reuben wrongfully takes a field from Simeon and sells it to Levi, and then Judah comes and contests Levi's title, Simeon then must not go and give evidence in favour of Levi, thinking that [if Levi retains it] it will be easier for him to recover it.  But if he has once testified that it belongs to Levi, how can he recover it from him?  — [We suppose] that what he will say [in evidence] is, I know that this field does not belong to Judah.  But cannot he recover it from Judah by means of the same proofs by which he recovers it from Levi?  — He says: It is easier for me to deal with the second [Levi] than with the first [Judah].  Or if you like I can reply that both [Simeon and Judah] have witnesses [to prove their title], and the Rabbis have laid down that in such cases the land shall remain in possession of its present owner.