Gemara

154a: R. Meir and Chachamim argue about whether a lender must validate a document (authenticate the signatures) if the borrower admits that he authorized it (but claims that he paid it). R. Meir says that he need not validate it;

Chachamim say that he must validate it. (If not, the borrower is believed to say that he paid, Migo (since) he could claim that it was forged.)

169b (Beraisa #1 - Rebbi): If Reuven says that he has a document and a Chazakah for his land, he uses the document to prove his claim;

R. Shimon ben Gamliel says, he brings witnesses of Chazakah.

(R. Avina): They argue about whether or not the lender must validate a document if the borrower admits that he authorized it. Rebbi holds that he need not validate it. R. Shimon ben Gamliel holds that he must validate it.

Question (Beraisa #2 - Rebbi): If a lender and borrower are holding a loan document, and each claims 'I dropped it', we validate it;

R. Shimon ben Gamliel says, they divide it.

(Rava): The document was not validated. The borrower admits that he wrote it. Rebbi requires the lender to validate it. An invalidated document is worthless. If we honor it only due to the borrower, he says that it was paid! R. Shimon ben Gamliel holds that the lender need not validate it.

Answer #1: We must switch the opinions.

Answer #2: We need not switch the opinions. In Beraisa #1, they argue about whether one must justify all his claims. Rebbi holds that he must validate the document and bring witnesses of Chazakah. R. Shimon ben Gamliel says that it suffices to bring witnesses of Chazakah.

Kesuvos 19a (Rav Huna): If David agrees that a document (that says that he owes Reuven) was written properly, Reuven need not validate it.

Rav Nachman (to Rav Huna): You deceive people! If you hold like R. Meir, say that the Halachah follows R. Meir! (Saying the law in your own name implies that all agree to it!)

Rav Nachman: In such a case, I tell Reuven to validate it first. (If not, we believe David due to the Migo.)

(Rav Yehudah): If he says that the document is Amanah (the borrower trusted the prospective lender and gave to him the document before the loan was given), he is not believed.

(Rava): He means that the borrower claims that it is Amanah. He is not believed for the reason of Rav Huna.

Rishonim

Rif (Bava Metzia 3b): The Halachah follows Rebbi, who says that the lender must validate it.

Nimukei Yosef (Bava Basra 72b DH Omar): A borrower is believed to say that a document was conditional only before the lender validated it. He is believed Migo he could say that he never sold or borrowed. Even though the claim is about money and the Migo is about the sale, and we hold that one is not believed about a matter due to a Migo he could claim about another matter, since he claims the money for the field, this is like one matter. There is a Chazakah that witnesses sign only if the parties are adults. This applies to a validated document. If it is not validated, he is believed to say that he was a minor, Migo he could say that he does not recognize the document, and it is a forgery. Normally, it is a Safek whether we rely on Migo against a Chazakah. Here we surely say the Migo. This is because not all Chazakos are equal, or because we leave money in its Chazakah.

Rambam (Hilchos Malveh 14:5): If Reuven brought against David a loan document that he cannot validate, and David says 'yes, I wrote this document. However, I paid it' or 'it was Amanah' or 'I wrote it to borrow, but I still did not borrow', or a similar claim, since he could have denied that anything happened, and it was validated through his admission, he is believed to swear Heses and exempt himself. If afterwards the lender validated it in Beis Din, it is like any document.

Poskim

Shulchan Aruch (CM 82:1): If Reuven brought against David a loan document, but he cannot find witnesses to validate it, and David admits that he wrote it, but says that he paid it, he is believed.

Taz: The borrower is believed only if he says that he paid Toch Kedei Dibur within admitting that he wrote it. If not, this is a retroactive Migo, and he is not believed.

Shulchan Aruch (ibid): Even if there is Ne'emanus in the document (it says that the lender is believed to say that it was not paid), it does not help.

Beis Yosef (DH Malveh): Sefer ha'Terumos says that presumably, even for a document with Ne'emanus, the borrower is believed to say that he paid. Since the lender did not validate it, the Ne'emanus does not help. An unvalidated document is (worthless,) like a shard. If you will rely on the borrower (who says that it is valid), he says that he paid! Ba'al ha'Itur says that the borrower is not believed in such a case due to a Migo; I disagree. The Tur holds like Sefer ha'Terumos.

SMA (2): Even when there is Ne'emanus, there is a Migo to say that the entire document is a forgery.

Shulchan Aruch (ibid): The same applies to any claim that would invalidate the document, e.g. 'it was Amanah' or 'I wrote it to borrow, but I still did not borrow', or 'it was conditional on a Tanai and the Tanai was not fulfilled.'

Beis Yosef (DH u'Mah she'Chosav v'Hu): Kesuvos 19a discusses Amanah (and says that the same applies). The Rambam adds one who says 'I wrote it to borrow, but I still did not borrow', i.e. and it fell from me and you found it. The Tur adds one who says that it was conditional, and the Tanai was not fulfilled.

Shach (1): Gidulei Terumah says that Tosfos (19b DH Omar Rav Nachman) and 19a (DH Eima) holds that a claim of Amanah is unlike a claim that he paid. Really, it is clear from the Gemara that Amanah is like a claim that he paid. No one disagrees.

Ketzos ha'Choshen (2): Hagahos Maimoniyos (Teshuvos Sefer Mishpatim 9) disagrees. He holds that just like witnesses are not believed to say that it was Amanah, for witnesses do not sign on Amanah, similarly the borrower is not believed to say this.

Shach (2): It seems that he is believed even if he says that the witnesses knew that it was Amanah or that he wrote it to borrow, but he still did not borrow. Even though he says that the witnesses are Resha'im, he is believed due to the Migo. A proof is from the Nimukei Yosef and Rema, who say that he is believed even if he says that he was a minor, even though this means that the witnesses are Resha'im, for they are obligated to check that the parties are adults. The Rashba says so, and Ba'al ha'Itur and Sefer ha'Terumos say so about a predated document. Witnesses are not believed to say that they signed on Amanah or that the litigant was a minor, for this would make them Resha'im; one cannot make himself a Rasha, and we split their words. We believe that it is their signatures, but not that it was Amanah. Here the borrower says that it was Amanah and we cannot validate the document, and the witnesses are not here, so he is believed due to the Migo. Therefore, if witnesses will come later and say that they signed but it was Amanah, the document is validated.

Gra (2): All the more so he is believed to say that it was conditional, for even the witnesses are believed to say that there was a Tanai. The borrower is believed about this even if the document was validated (Sa'if 12).

Note: There we say that he can force the lender to swear, and if the lender admits that there was a Tanai (to exempt the borrower), he must prove that it was not fulfilled.

Rema: If he says 'I was a minor at the time it was written', he is believed.

Gra (3): Even though witnesses are not believed about this, (the borrower is, for) this is no worse than saying 'it was Amanah.'