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.)

174b (Rav Huna): If a Shechiv Mera was Makdish all his property, and later said '100 Zuz in my possession really belongs to Ploni', he is believed. There is a Chazakah that people do not scheme to cheat Hekdesh.

Objection (Rav Nachman): Do people scheme to cheat their children?! Rav and Shmuel taught that if a Shechiv Mera said '100 Zuz in my possession really belongs to Ploni', we give to Ploni only if he said 'give it to Ploni.'

(This is not a scheme to cheat his children. Rather,) one does not want people to think that his children are wealthy, so we suspect that he is lying. Likewise, perhaps the one who was Makdish did not have Ploni's money, but he did not want people to think that he himself is wealthy!

Answer: Rav Huna discusses when Ploni has a document.

Question: This implies that Rav and Shmuel discuss when Ploni does not have a document. If so, when he says 'give', why do we give? This is (like a creditor trying to collect from buyers) a loan without a document. Rav and Shmuel taught that a loan without a document is not collected from heirs or buyers!

(Rav Nachman): Rather, Rav Huna discusses a validated document. Rav and Shmuel discuss a document that is not validated. If he says 'give', this is like validating the document. If he did not say 'give', he did not validate it.

Bava Kama 106a (Rav): If Reuven denied owing money to Shimon and swore, and later, witnesses said that he owes Shimon, he is exempt. "The owner will take, and he (the defendant) will not pay" - once the owner received (heard) an oath, the defendant need not pay.

Rami bar Chama (to Rav Nachman): You do not hold like Rav. What forces you to (explain the Mishnah in such a way to) answer for him?

Rav Nachman: I just explain how Rav must learn the Mishnah.

Rishonim

Rif (Shevuos 22a): When the Gemara says that one cannot collect with a document, it is Batel. The borrower swears Heses that he does not owe.

Rebuttal (Ran, Dibur Rishon): The document is not Batel. We do not tear it. It is improper to tell the borrower to swear as long as the lender has the document. If the lender wants the borrower to swear, he must return the document first.

Nimukei Yosef (Bava Basra 82a DH Shechiv): In the case of the Shechiv Mera who was Makdish his property and later said that 100 belongs to Ploni, the Ramban says that he was healthy when he was Makdish. Now he is a Shechiv Mera, and he says that 100 belongs to Ploni. We infer that we collect from orphans only with a validated document. If the borrower is here and he does not claim that the document was forged, we do not claim for him. If he claimed that the document was surely forged, and afterwards the lender validated it, the borrowed is not believed afterwards to bring a proof that he paid. If one says 'it is a forgery. I did not borrow', it is as if he said that he did not pay. His admission is like 100 witnesses.

Nimukei Yosef (Bava Basra 82b DH Omar): Ploni collects only if the Makdish validated the document. Meforshim learn from here that even from heirs, one does not collect until he validates the document. This is unlike the Ri, who requires validation only when the defendant makes a Vadai claim, but not against heirs. The Ri is wrong.

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', 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 that he cannot find witnesses to validate it, and David admits that he wrote it, but says that he paid it, he is believed. If afterwards the lender validated it in Beis Din, it is like any document and he collects with it.

SMA (5): He can use the document to collect from one who bought in the interim. The buyers caused their own loss. They should not have bought, lest he validate the document. Now that he validated it, this shows retroactively that it was valid.

Bach (DH Kosav): Why should one think that he cannot collect Meshubadim? The Chidush is that he collects even from property sold after the document was seen in Beis Din and the lender was unable to validate it. One might have thought that in the interim, the document was like a shard.

Bach (DH Aval): Do not say that after the borrower swears, the lender collects. Beis Din would not impose an oath if it is possible that afterwards the lender will validate the document, and the oath was Levatalah. The Rambam himself (Hilchos Malveh 14:15) says that if the borrower says that he paid, and the lender asks him to answer, he says 'you still have a document!' He does not swear until we disqualify the document (CM 75:24). Only a fool would swear and leave the document with the lender! Letter of the law he need not swear as long as the lender has a document against him, like the Ran (above, also in Beis Yosef 58:3 DH v'R. Chananel) says. Rather, we say that it is like any document only before the borrower swore. Once he swears, the document is Batel. He is believed through his oath that it was paid. However, if the borrower claimed that if was forged, surely the lender need not return it after the borrower swears.

Rebuttal (Shach 4): Even after the borrowed swears, the lender need not return the document. The Rambam says 'he is believed to swear Heses and exempt himself. If afterwards the lender validated it in Beis Din, it is like any document. Also Sefer ha'Terumos says so. If the borrower says 'I will not swear until you return the document', we do not heed him. The lender can say 'swear, for perhaps I cannot validate it. I will keep the document, for perhaps I can validate it.' The oath was not Levatalah, for it helps if the lender cannot validate it. The Bach's proof from the Rambam (14:15) and CM (75:24) is invalid. There, he claimed without a document, and the borrower said 'I know that you have a validated document against me. Why should I swear Levatalah?!' Here is more like 75:25. When the lender has witnesses, he can make the borrower swear immediately, for perhaps he cannot bring the witnesses now. Also here, perhaps he will be unable to validate the document. There is no proof from the Ran, for the Ran discusses one who wants to keep the document in order that he can seize even in front of witnesses and keep the property. We should not force the borrower to swear and let the lender keep this ability. The borrower can say 'just like you are Muchzak in your document, I am Muchzak in my Shevu'ah; I will not swear until you return my document, so you will not be able to seize. Here, if he validates the document he will be able to collect without seizing. Therefore, the borrower must swear even though the lender keeps the document.

Taz (75:25): If the borrower refuses to swear until the lender tears the document, the lender can say 'now I do not have witnesses. Perhaps later I will find witnesses to validate the document!' What can the lender do now? This is unlike a lender and borrower who argue about the value of a security. There, it helps for the lender to swear first (to avoid a false oath). Here there is no solution. The Ran discusses a document with which one may not collect.

Gra (4): The lender collects afterwards whether the borrower swore Heses for total denial, or mid'Oraisa for partial admission. (The Halachah follows Rav Nachman in monetary laws, and) Rav Nachman disagrees with Rav (who says that a Shevu'ah permanently exempts).