Gemara

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 the document 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 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.

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.

Kesuvos 87b - Question: If she admits that she is not entitled to the amount written in her Kesuvah, must she swear about what she says that she is entitled to, in order to receive it?

If this is like admitting to partial payment, she must swear. Or, perhaps it is different, for she does not admit to any partial payment?

Answer (Beraisa): One who admits that she is not entitled to the amount written in her Kesuvah receives the rest without an oath. If the Kesuvah says 1000, he says that he paid it, and she denies this but she admits that she accepted that it be only 100, she is paid without swearing.

Question: Why does she receive (100)? Her document is invalid!

Answer (Rava brei d'Rabah): She says that he authorized the witnesses to write a Kesuvah for 1000, and she promised to claim only 100.

Rishonim

Rif (Bava Basra 16b): The forged document was not a total forgery. If it were, Rabah would agree that it is totally invalid! Rather, it was Amanah (the borrower gave it to the lender before borrowing, and trusted him not to demand payment before he lends the money) He could have validated it in Beis Din.

Rosh (Bava Basra 3:12): This is like Rav Hai Gaon and R. Chananel. The Rashbam says that the same applies to a total forgery that he could have validated, i.e. witnesses signed Sheker for him, or he himself forged so well that witnesses thought that it was their signatures. This is reasonable, for Amanah is as Pasul as a forgery. Surely, Rabah would not have honored the document without validation, for Reuven claimed that it was forged. The Rashbam says that once he admitted that it was a forgery, Rav Yosef considers this like a Migo against witnesses, for it is as if witnesses testified that it is Pasul. He holds that 'I bought it from you' and 'this is my document' are like one claim, for his entire claim is based on the document. This is wrong. His primary claim is 'I bought it from you.' The document is a mere proof.

Rosh (Shevuos 6:19): Why do we need a Migo to say Sitrai? Why is this different than saying 'you did not pay me at all'? It seems that Sitrai is not such a clear claim, for there is a Chazakah that one pays a loan with a document before paying a Milveh Al Peh. Therefore, we require a Migo that he did not pay at all, which is a strong claim. Ri ha'Levi says that if witnesses do not know why he gave the money, the lender can say that it was Sitrai, Migo he could say that it was a gift. Sitrai is not considered taking through a Migo. The document is valid in any case. The Migo enables him to keep the money he received without invalidating his document. The Ramban says that this is not a Migo. If we do not believe that he paid a Milveh Al Peh before a loan document, surely he did not give a gift to the lender before paying his loan! I agree.

Rambam (Hilchos To'en 15:9): Reuven had witnesses that a field was his. Shimon showed a validated document of purchase from Reuven. Reuven said that it was forged. Shimon said 'yes, it was forged, but I had a proper document, and I took this to scare him into admitting that he sold it to me.' Since he could have persisted that his document is valid, he is believed. We take the field from Reuven; Shimon must swear Heses.

Rambam (Hilchos Malveh 14:6): Shimon showed a validated loan document against Reuven. Reuven said that it was forged, or was Amanah. Shimon said 'yes, but I had a proper document.' Even though he ruined his own document, for he could have denied that it is forged, he does not collect with it. Reuven swears Heses and he is exempt, for the document is worthless.

Magid Mishneh: The Rambam explains that in the episode n Bava Basra 32b, the document was validated. This is like the Ge'onim and R. Yonah. The Ramban and Rashba say that the document was not validated when he claimed his Migo, therefore, we do not honor his claim. Had he validated it beforehand, he would have collected.

Ramban (Bava Basra 32b DH Amai): Shimon's document was not validated. Rabah ruled that he will be believed through Migo only if he validates it. Rav Yosef said that since he admitted that the document is worthless, we do not validate it. He was foolish to admit before validating it, and loses. The Gemara was unsure whom we follow, and ruled that we leave the money or land where it is. In Kesuvos (87b we ask 'how can she collect with a Pasul document?' We ask according to the conclusion here (like Rav Yosef regarding money). Alternatively, we asked according to everyone, for we understood that she cannot validate her document. In Shevuos (42a) we believe one to collect through a claim of Sitrai. This is no proof that one may take through a Migo. Sitrai is regarding the money he received and is Muchzak in, just it helps to collect the document

Tosfos (Bava Basra 32b DH v'Hilchesa): Normally, we do not take through a Migo. We do take through Sitrai, for there is a document.

Rashba (Teshuvah 100, attributed to Ramban): Even though one may not take through a Migo. Sitrai works because it is as if the bearer of a document is Muchzak in the money. It is like a Migo to avoid paying.

Poskim

Shulchan Aruch (CM 83:4): Shimon showed a validated loan document against Reuven, who said that it was forged, or was Amanah. Shimon said 'yes, but I had a proper document.' Reuven swears Heses and he is exempt.

Beis Yosef (DH To'an): The Tur discusses when Reuven said that it was Amanah or paid. All the more so, Reuven swears Heses and he is exempt if he said that it was forged.

SMA (9): The Tur did not discuss a forgery, for it is rare that one claims that a validated document is a forgery.

Drishah (5): Here the Tur teaches that even though the document is validated, if Reuven said that it was Amanah or paid and Shimon admits, Shimon is not believed through a Migo. In Siman 146 (regarding a document of sale) the Tur teaches the Chidush that even if it was not validated, if Reuven validates it later he is believed through a Migo.

SMA (10): Even those who say that we may take through a Migo (Rema 82:12) admit that here we do not, for Shimon admits that his initial claim was with a false document. Migo is only to prove that his claim was true. Why does Ir Shushan say that the reason is because one may not take through a Migo? Meforshim argue about taking through a Migo, but the Shulchan Aruch's law is explicit in the Gemara!

Rebuttal (Shach 7): Ir Shushan is correct. Tosfos said that Rav Yosef rejects the Migo because his initial claim was with a false document. This is unlike Rav Idi, who ruled like Rabah regarding land! Rather, even if the initial claim was false, Migo helps to avoid paying, but not to take. All the Poskim rule like this. The Gemara brings the Shulchan Aruch's law, but it did not say that the document was validated. The opinion that we may take through a Migo holds that it was not validated, but if it were, he would collect. The Rema relied here on what he wrote in the previous Siman. The Darchei Moshe explicitly says that the Ramban and Rashba argue here.

Question (Gidulei Terumah, brought in Shach 9): The Ramban and Rashba hold that Shimon is believed, because his document was validated. They hold that regarding land he is believed even though his document was not validated when he admitted. This is difficult. We say in Kesuvos (87b) that one cannot collect with an invalid document!

Answer (Shach): They hold that after he validates his document, he is believed through his Migo.

Shach (87:29): Maharshach asked that even Tosfos, who holds that one may not take through a Migo, proved from Sitrai that we may take when there is a document! Sefer ha'Terumos answered that this is only when there is a Kosher document, and a document on Tanai is like a Pasul document. I disagree (87:28). Also the Ramban (see Teshuvas ha'Rashba above) allows one with a Kosher document to take through a Migo. According to this, according to the Mechaber (58:2, 70:4) one cannot claim Sitrai with a Milveh Al Peh even if he stipulated that he must be paid in front of witnesses.

Ketzos ha'Choshen (82:10): If one admitted that her Kesuvah was partially paid, she swear. R. Yonah (Shitah Mekubetzes Kesuvos 87b) says that she is not believed due to a Migo, for it is to take. R. Yerucham asked that one with a Kosher document is believed to take! Perhaps R. Yonah disagrees. The Ramban rejected the proof from Sitrai. It is not a Migo to take, rather, to keep the money he received without disqualifying his document. Or, we take with a Migo only if the document can collect from Meshubadim; one cannot do this with a partially paid document (SMA 55:6).