Gemara

(Mishnah): If two are holding a garment, and each claims 'it is mine', each swears that he does not own less than half, and they divide it.

If Reuven says 'it is all mine' and Shimon says 'it is half mine', Reuven swears that he does not own less than three quarters, and Shimon swears that he does not own less than one quarter. Each gets like he swore.

Bava Basra 32a: Reuven challenged Shimon 'what are you doing on my field?' Shimon showed his document of purchase. Reuven said 'it is a forgery!' Shimon whispered to Rabah (who was judging the case) 'indeed, it is a forgery, but I had a proper document and lost it.'

Rabah: Shimon is believed. If he wanted to lie, he could have insisted that it is a proper document!

Rav Yosef: We cannot rule in Shimon's favor due to the document, for we now know that it is a forgery!

(In another case,) Levi asked Yehudah to pay the money he owes, and showed the loan document. Yehudah said that it is a forgery. Levi whispered to Rabah 'indeed, it is a forgery, but I had a proper document and lost it.'

Rabah: Levi is believed, for he could have insisted that it is a proper document!

Rav Yosef: We cannot rule in his favor because due to the document, for we now know that it is a forgery!

(Rav Idi bar Avin): The Halachah follows Rav Yosef regarding money. We leave the money where it is (with Yehudah). The Halachah follows Rabah regarding land. Shimon is now on the land, so we leave him there.

134b (Rav Yosef): Rav Yehudah taught that if one said 'this is my son', he is believed to exempt his wife from Yibum, Migo he could exempt her by divorcing her. Therefore, a man is believed to say that he divorced his wife, because he could divorce her.

Kesuvos 15b (Mishnah): R. Yehoshua agrees that if Reuven says to Shimon 'this field was your father's; I bought it from him', he is believed. The one who forbids is believed to permit (without his admission, Shimon had no claim);

(Mishnah - R. Gamliel and R. Eliezer): A Kalah was found to have no Besulim (tokens of virginity). She says that she was raped after Kidushin. He says that (perhaps) she had Bi'ah before the Kidushin, and does not have a Kesuvah of 200. She is believed;

R. Yehoshua says, she is not believed (even though she has a Migo, to say that she is a Mukas Etz (she lost her Besulim through an injury).

He accepts the Migo about the field, for there, the ox is not slaughtered in front of you (Shimon had no claim until Reuven spoke up; Alternatively, Reuven did not need to say that Shimon's father once owned it.)

Shevuos 42a: Reuven claimed a Maneh from Shimon, and showed the loan document. Shimon claimed that he paid him. Reuven said 'that was Sitrai (you paid a different debt).'

The Halachah is, if he paid without witnesses, Migo that Reuven would be believed to say that he was not paid, he is believed to say that he collected a different debt.

Rishonim

The Rif (Bava Metzia 1a) brings our Mishnah.

Rosh (Bava Basra 3:13): Rivam says that one cannot take with a Migo. This is true, but the Gemara connotes that this was not Rav Yosef's reason. Rather, Rav Yosef holds that if one must lie first, this is not a Migo. We can resolve the Rivam with the text of the Gemara. Rav Yosef asked 'do you consider Shimon to be Muchzak due to the Migo? You cannot rely on the document, for it is worthless. We do not use a Migo to take!' The Halachah follows Rabah regarding land. Once he said that it is a good document, he was Muchzak in the land. After he admitted, the Migo keeps the land in his Chazakah. Regarding money, even after his initial claim he was not Muchzak in it, for he still needed to collect it.

Rambam (Hilchos To'en 9:8): If Reuven says 'it is all mine' and Shimon says 'it is half mine', Reuven swears that he does not own less than three quarters, and Shimon swears that he does not own less than one quarter. Each gets like he swore.

Tosfos (Bava Basra 32b DH v'Hilchesa): The Rashbam says that we are in doubt whom the Halachah follows (so we leave the land where it is). He must distinguish between a Safek due to contradictory testimony, and a Safek whom the Halachah follows. When witnesses argue about whether or not the seller was sane at the time, the testimonies cancel each other and we leave the property with the original owner. Here, it is unreasonable to say that since we are unsure, the original owner keeps it and the Machazik (who entered the land) must leave. This is difficult. Also, 'the Halachah is' connotes an absolute ruling, not merely how we conduct due to Safek. Even if one ruled like Rav Yosef (regarding land), we overturn the ruling. The Ri says that we follow Rabah regarding land because a Migo helps to keep land in the possession of its current owner. Normally, we do not take through a Migo.

Ramban (Bava Basra 32b DH Amai): Why isn't one believed to claim verbally, since he could (forge a document and) claim with a document? We say a Migo even if one must do an act first, e.g. divorce (134b)! Some say that one may not take through a Migo, and here Reuven had a Chazakah from his fathers. Rabah holds that this is not called taking through a Migo, for Shimon was in the land. Alternatively, Shimon was already there for three years, but if his document was invalidated, his Chazakah would be invalid. This is wrong. If so, surely this is a Chazakah to keep what he has, and he is believed. Rather, Shimon was there for less than three years. Those who say that one may not take through a Migo y learn from two holding a Talis. If Reuven claims all of it, and Shimon claims half, he gets only one quarter. He is not believed to get half, Migo that he could have claimed all of it. How will Rabah explain this? I say that it is no proof, for he is believed to receive only half of what he claims. However, we believe a woman to collect a Kesuvah of 200, Migo she could say that she is Mukas Etz.

Poskim

Shulchan Aruch (CM 82:12): If the borrower says that they stipulated that he will be exempt if he fulfills a certain Tenai, and the lender agrees but says that he did not fulfill it, the lender must bring a proof. If he cannot, the borrower swears Heses and is exempt.

SMA (44): The Rivash (336) says that even the opinion that does not allow taking through a Migo, says Migo against land. We do not say that land is in the Chazakah of its owner.

SMA (45): This is from Sefer ha'Terumos. He applies Migo (to take) only when the claimant has a good document. The lender admits that there was a Tenai, and if the Tenai was fulfilled, the document is invalid, so it is not a proper document. This is like one who admitted that his document was forged. One with a proper document can take through a Migo, e.g. Sitrai. Sefer ha'Terumos and the Shulchan Aruch say that one cannot take through a Migo because he considers a proper document as if it was already cld, so this is not called 'to take.'

Bach (19 DH v'Kashya): R. Gamliel believes a woman to say that she was raped and her Kesuvah is 200, Migo she could say that she is a Mukas Etz! Perhaps that is because she is Vadai, and her husband is not. Why didn't the Gemara say that R. Yehoshua admits about the field, because that Migo is only to keep? Perhaps it could have answered this also.

Shach (Dinei Migo, Sof Siman 82, 1): Even R. Yehoshua would allow taking through a Migo when she is certain and he is Safek, if not that the ox is slaughtered in front of you.

SMA (46): The Rivash (336) says that all the Acharonim say that we say Migo to take. Why did the Shulchan Aruch (some texts - Ir Shushan) omit this?

Rebuttal (Shach 28): Also the Tur says so, but I disagree. The lender says that the document is Kosher! This is like Sitrai. Even the opinion that does not allow taking through a Migo, agrees that one may take through Sitrai. Amanah (the lender received the document before lending) is invalid (CM 83:4) only if he did not lend. If he says that he lent, he collects. Amanah is the ultimate Tenai! It is worse than a Tenai, for we believe witnesses who say that the document had a Tenai, but not if they say that it was Amanah. The Ramban and Rashba say that one is believed through a Migo even if he admits that his document is Pasul.

Shach (Dinei Migo, Sof Siman 82, 15): The Mordechai learns from Kesuvos that we may take through a Migo when there is a Vadai claim (against a Safek claim). Why is this a proof? Perhaps there we may take because there is a Kesuvah document! Surely he agrees that one with a document may take through a Migo, like Rishonim proved from Sitrai. Even if she has no Kesuvah document, an act of Beis Din is as if one has a document. It seems that a lender may take through a Migo when he has a document and he knows that the borrower has his money. She never lent to her husband. Therefore, if he were Vadai, she would not collect through a Migo. Even if he wrote a Kesuvah, he could say that he mistakenly thought that she was a Besulah. The Migo would not help her, since he owed her only due to the Kesuvah. Therefore, this is a proper source that a Vadai may take from a Safek through a Migo.

Gra (38): She does not take through a Migo due to the document. Since he claims that she was not a virgin, it is as if there is no Kesuvah, like we say regarding a Tenai in a document.

Rema: The lender is not believed with a Migo (that he could have denied that there was a Tenai), for we do not take money with a Migo. Some disagree and say that we take through a Migo.