Gemara

(Beraisa): If one was Motzi a loan document that was not yet paid, he is liable. If it was already paid, he is exempt.

R. Yehudah obligates even after paying it, for he still needs it.

(Abaye): They argue about whether or not the lender must validate [authenticate the signatures] a document when the borrower agrees that he authorized it [but claims that he paid it;

The first Tana requires validation.

R. Yehudah does not require validation.

Question: Why does it say that they argue about after it was paid? According to Abaye, they argue about whether or not we believe that it was paid!

Answer: Indeed, the first Tana distinguishes between whether or not the borrower says that he paid

(Rava): All agree that the lender must validate the document. They argue about whether or not we [force a borrower to pay when the lender cannot find the loan document, and] write a receipt.

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: In such a case, I tell Reuven to validate it first.

(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.

Bava Basra 154a: R. Meir holds that a lender need not validate a document if the borrower admits that he authorized it [but claims that he paid it]. Chachamim disagree.

Rishonim

Rif (Bava Metzia 3b): The Halachah is that the lender must validate it.

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.

Magid Mishneh: In Bava Basra, most Meforshim rule like the Rambam, that even if the borrower admits that he authorized a document, the lender must validate it. Also the Ramban, Rashba and Rif say so.

Rambam (Hilchos Gerushin 12:2): If a woman shows a Get and says that she was divorced, we permit her to marry even though she did not validate it.

Rebuttal (Ra'avad): I say that she must validate it even if there is no protest.

Rambam (3): If the husband comes and protests and says that he did not give it, rather, it fell from him and she found it, he is not believed, since he agrees that he wrote it and she is holding it. If he says that he gave it on condition, or it was a deposit, or he never wrote it and it was forged, she must validate the signatures or bring witnesses who saw it given. If she cannot, she may not remarry.

Magid Mishneh (Hilchos Gerushin 12:2): The Rambam says that the husband is not believed to say that it fell from him and she found it, and she need not validate it. He learned from Gitin 64a. Rav Chisda says that if a third party is holding the Get, the husband is not believed to say that it was a deposit. All the more so, he is not believed to say that it fell when she is holding it. Isurim are unlike monetary laws. Even Rav Huna, who argues about a third party, agrees when she is holding the Get. Surely, when he claims that it is forged, she must validate it. A woman holding a Get is like a Get brought in Eretz Yisrael. The Shali'ach need not say Befanai Nichtav Befanai Nechtam, and if there is a protest, she validates it.

Ketzos ha'Choshen (CM 46:1): The Tumim (46:1) brought the Lechem Mishneh, who explains that regarding money, one is believed through a Migo to say that it fell. Regarding a Get, which is Isurim, we are not concerned for this claim, even through a Migo. If so, how did Rava learn money from Isur in Gitin 24b? I answer that we are concerned for Nefilas Rabim (when it could have fallen from any of a large number of people), but not for Nefilas Yachid (there is only one person from whom it could have fallen).

Poskim

Shulchan Aruch (CM 46:1): If Reuven brought against David a loan document, before he brings witnesses to validate it, David can say 'it is forged. I never authorized it.'

Gra (DH ha'Motzi): Even R. Meir agrees about this.

Beis Yosef (DH u'Mah she'Chasav Lo): Rav Nachman holds that as long as the document was validated only through the borrower, he is believed to say that he paid, for the mouth that forbade (in this case, admitted that he authorized the document) is the mouth that permitted (said that he does not owe). It is known that the Halachah follows Rav Nachman in monetary laws.

SMA (1): Here the Tur mentioned only a borrower who says that it was Amanah. In Siman 82, he adds one who says 'I wrote it to borrow, but I still did not borrow', like the Mechaber says here. The Beis Yosef explains that he prepared the document in case he will need to borrow, and trusted the lender not to claim payment with the document before lending. The case of 'I wrote it to borrow, but I still did not borrow', is when it fell from him. The witnesses are not believed to say that it was Amanah, for one may not sign it. They are not believed to say that they are Resha'im. They are believed to say that it fell from the borrower, for this does not incriminate themselves. One may write a document for a borrower not in front of the lender if he intends to borrow immediately. Perhaps a reason occurred why he could not lend immediately. This is unlike signing with intent for Amanah.

Question (Ketzos ha'Choshen 1): This is difficult, for the Tur and Shulchan Aruch wrote (EH 152:9,10) that if a woman shows a Get, we permit her to marry. If the husband comes and protests and says that he did not send it, rather, it fell from him, he is not believed, since he agrees that he wrote it and she is holding it. If he says that he gave it on condition and it was not fulfilled... (she must bring witnesses). This shows that he is not believed to say that it fell, even though he has a Migo to say that it was forged. Even though if he says that there was a Tanai or he gave it for a deposit, he is believed through a Migo, Chachamim do not believe a claim that it fell, even through a Migo. This is also difficult due to Siman 50, regarding a document that says 'I, Ploni, borrowed 100 from you.' If there are no witnesses that it is Ploni's handwriting, he is believed to say that he paid, Migo he could say 'I did not borrow from you, rather, from Almoni. It fell from him, and you found it.' Through a Migo, he is believed to say that it fell!

Answer (Ketzos ha'Choshen): We can distinguish between Nefilas Rabim (Ploni's document could have been from anyone) and Nefilas Yachid (the husband). Even though we do not distinguish between falling of a Rabim from an individual (Bava Basra 172b), like Rabah, that is when there is no Migo. Where there is a Migo, we can distinguish. Here, when he says that he wrote to borrow but never borrowed, and he himself dropped it, this is like the Get!

Shulchan Aruch (ibid.): Even if he admits, and says 'yes, I authorized it, but I paid it', or 'it was Amanah', or 'I wrote it to borrow, but I did not yet borrow', or similar claims, since if he wanted he could say 'I never authorized it', for it was validated only through his mouth, he is believed. He swears Heses, and he is exempt. If the lender validated it afterwards, it is like all documents. See below 82:1.

Gra (DH v'Im): We learn from Rav Nachman (who told the lender to validate the document). We must say that the borrower already claimed that he paid. If not, Rav Nachman acted like a lawyer (suggesting to the borrower how to claim. Avos 1:8 says not to do so.)

Gra (Likut DH v'Im, 82:4): This is even if he already swore, like it says in Bava Kama 106a. Rav holds that after one swore, he is exempt even if was come later, and Rav Nachman holds unlike Rav. (The Halachah follows Rav Nachman in monetary laws.)