Explanation #1 (Rashbam): If he was Muchzak three years, and he claims "I bought it from Ploni, who bought it from you in front of me", he is believed through a Migo. If he wanted, he could have said "I bought it from you, and ate the years of Chazakah";

Alternatively, if he has witnesses that Ploni was Machazik in the field one day [he is believed].

Proof (Rashbam, for himself): We learn from the one who lived in Kashta (a place), in an attic, below (41b).

Objection and Explanation #2: This is correct when the protester has witnesses that the land was his. However, the case here connotes that the protester had no witnesses, since he said "you admitted..."

If [the Machazik] would have claimed "[Ploni] bought it from you in front of me", or "he lived in it one day", he would be believed, even though he did not have three years of Chazakah", Migo (since) if he wanted, he could say "it was never yours."

However, now that he said "I bought it from Ploni, who bought it from you", and he admitted that it was [initially] the protester's...

[Now] he is not believed to claim "he bought it from you in front of me", or "in front of me, he lived there for a day", through a Migo, that if he wanted, he could have said from the beginning "it was never yours", for we do not apply a Migo retroactively.

Proof #1: Below (41b), regarding the one who lived in Kashta, in an attic, for four years, and said "I bought it from Ploni, who bought it from you", he came in front of R. Chiya;

Citation (41b - R. Chiya): Bring witnesses that he lived there even for one day, and I will establish it in your hand.

Inference: If he cannot bring witnesses, he is not believed to say "in front of me, he lived there for one day", Migo (since) if he wanted, he could have said "I bought it from you";

Since he already admitted that he did not buy it from him (the protester), he no longer has a Migo;

He should not be believed about what he says now [just] because he could have lied initially, before he admitted, because he did not think that he will need this claim.

Implied question: Rav said there "I could see that (R. Chiya held that) if the Machazik would claim "in front of me, Ploni bought it from you", he would be believed through a Migo, that if he wanted, he could have said 'I bought it from you." (This implies that even now, he is believed to say so!)

Answer: He did not mean that he is still believed with this claim. Rather, had he initially claimed so before he admitted [he would have been believed].

Likewise, had he initially claimed "in front of me, he lived there for one day", he would have been believed.

Proof #2: In Sanhedrin (31a), a woman held a document (the lender and borrower entrusted her with it) and said "I know that this document was paid..." Rav Nachman did not believe her;

Rava asked that [she should be believed, for] if she wanted, she could have burned it! [Rav Nachman answered that] once a document was established in Beis Din, we do not say [that one is believed to disqualify it, Migo] he could have burned it.

Inference: Even though before we saw it in her hand, she would be believed to say that it was paid, Migo she could have burned it, now that it was established in Beis Din, i.e. we saw it in her hand, she is not believed, for there is no longer a Migo here.

You are forced to say that now she needs this claim that she says to exempt the borrower.

Disclaimer: The Rashbam explained there that "established in Beis Din" means that [the signatures] were validated. (If so, there is no proof that one is not believed through a retroactive Migo.)

Rebuttal (and defense of Proof #2 - Ri): How does [validation] help? In any case, she could have burned it if she wanted!

Proof #3: In Shevuos (38b), [Ploni] said "you have 100 [Zuz] of mine", and [Almoni] agreed. The next day, [Ploni] said "give it to me", and [Almoni] said "I gave it to you. He is exempt;

The Gemara asks from this against the opinion that one who lends to his friend with witnesses, he must pay in front of witnesses. Here, since he claimed from him in front of witnesses [and Almoni admitted], he is like one who lent to him with witnesses, and it teaches that he is exempt!

Question: What was the question? Here is different. He is exempt due to a Migo, that if he wanted, yesterday he could have said "nothing ever happened" (I never owed you)!

Answer: Rather, surely in such a case we do not apply Migo.

Explanation #1 (Rashi in Kesuvos 109b): In the case of [David] who made [Shimon's] field a Siman (he said that it is one of the borders of an adjacent field), if he claimed and said "later, I bought it from Shimon, after I made it a Siman", [R. Yochanan taught that] he is believed, for ha'Peh she'Asar Hu ha'Peh she'Hitir;

Since [David] has witnesses that it was his, and it was stolen from him, and [Shimon] has a claim only based on David, who made it a Siman for him, since later he said "I bought it from you" [Shimon has no claim now].

Objection (Ri): This is wrong. In such a case, we do not apply Migo, like I explained. He is not believed now through what he could have claimed.

Explanation #2 (Ri): Rather, there [David] later ate years of Chazakah, after he made it a Siman.

Implied question: If he ate it the years of Chazakah, this is obvious! What is R. Yochanan's Chidush that he is believed? Our Mishnah teaches this! A Chazakah with a claim is a Chazakah!

Answer: One might have thought that he is not believed. Since [David] protested about it before he ate the years of Chazakah, and said that it is his, one does not normally buy it afterwards in such a case. [R. Yochanan] teaches that this is not so. (Rather, he is believed.)

Support: Similarly, the case brought after this discusses when he ate the years of Chazakah.

Assertion (Rashba): In our case, if the Machazik would have brought witnesses that Ploni lived there for one day, we would establish the land with him (the Machazik).

Implied question: There is no Migo (for he did not eat the years of Chazakah)!

Answer: In any case, we claim for the buyer, since the seller would have been believed to say "I bought it from you", Migo he could have said "it was never yours."

Implied question: This buyer knows that it was his (the protester's). He is like one witness! (Why should we believe a Migo against a witness?)

Answer: In any case it is a Migo, like it connotes below (33b) regarding the ingot of R. Aba. (One witness testified that Reuven grabbed an ingot from Levi. Reuven said 'yes, I grabbed it. It is mine!')

If we would not say "he is obligated to swear, but he cannot", he would be believed to say "I grabbed my own", Migo he could have said that he did not grab, and he would contradict the witness.

Here, he is not obligated to swear, for we do not swear about land.

Support: We find like this below (70b) regarding one who deposited with his friend with a document;

Even though we do not claim for orphans that Ones occurred, even so we claim for him "I returned it to you", since their father would have been believed with this claim, Migo that if he wanted, he could have said that Ones occurred.

Rejection (Ri): Here is unlike there (the deposit);

Here, if the seller wanted to win with this claim "it was never yours", the buyer (the Machazik) would not keep it, for he knows that it was his (the protester's, initially)!

Note: Ketzos ha'Choshen (146:11) cites Rishonim who hold that since the buyer has a Migo, this shows that his claim is true. Therefore, the Machazik receives the field. The Ri disagrees. We believe the claim only as long as the Migo applies. Since the Machazik already admitted that the Migo is not true (the protester initially owned it), the Migo does not apply for him.

Opinion #1 (R. Chananel): I saw from Raboseinu, the Ge'onim, that the Machazik cannot take the [original] owner of the land to Beis Din. (The owner can say, you agree that you did not buy it from me. You have no claim against me!) Rather, the seller goes for judgment with him (the original owner).

Rebuttal (Rashba): The first (the seller) sold to the second (the Machazik) all rights that come to his hand. [The original owner] cannot dispel [the Machazik] and say "you have no claim against me."

Support: Below (41b), we find regarding the one who lived in Kashta, in an attic, that he [the owner] cannot say "you have no claim against me."

Rather, if [the Machazik] could find witnesses that the man [from whom he bought it] lived there for one day, R. Chiya would have established the land with him.

Explanation #1: [The original owner says] "I have witnesses that he is a thief." Even if he lived there one day, we hold that a thief has no Chazakah.

Explanation #2 (Rashbam): [The Machazik] has no witnesses that the seller lived there even one day. If he did, [the original owner] would not be able to claim that Ploni is a thief.

Inference: He explains that [the original owner] does not have witnesses that Ploni is a thief. Rather, he claims that Ploni stole it.

Rebuttal (Ri): [If so,] he should have concluded like we concluded above "didn't you admit that the land was mine, and you did not buy it from me?!..."

Explanation: However, if this were not common, even though he has witnesses that he ate for seven years, we would not believe him, for he contradicts them;

He admits that he ate only the years of Chazakah, i.e. three years.