Gemara

(Rav Papa): We do not (allow Reuven to) collect with a tainted document.

The case is, two said that Reuven asked them to forge for him.

Question: We do not suspect Yisraelim of forgery!

Answer: Since Reuven was trying to get a forged document, we are concerned that he forged it himself.

20b (Mishnah - Rebbi): If each of the Edei Chasimah (the witnesses who signed) recognizes only his own signature, we need another witness who recognizes both signatures to validate the document;

Chachamim say, this is not needed. A person is believed to verify his signature.

According to Rebbi, witnesses testify about their own signatures. According to Chachamim, they testify about the content of the document.

Bava Basra 167a: A document came in front of Rava. He and Rav Acha bar Ada were signed on it. He recognized his own signature, but knew that he never signed in front of Rav Acha! Rava coerced the bearer; he admitted to his forgery.

Rava: Granted, you were able to forge my signature. How did you forge Rav Acha's signature? His hand shakes!

The forger answered that at the time he held a rope that people hold when they cross a narrow bridge, or that he was standing on a bucket (and wobbling).

Makos 5b: A woman brought witnesses; they were found to be liars. She brought more witnesses, and they were found to be liars. She brought more witnesses, they were not found to be liars.

Reish Lakish: She is Muchzekes to bring false witnesses!

R. Elazar (and R. Yochanan): Even if she is Muchzekes to bring false witnesses, Yisraelim are not Muchzakim to agree to lie.

Rishonim

Rif: We do not collect with a tainted document. The case is, two men said that Reuven asked them to forge this document for him. Since he was trying to get a forged document, we are concerned that he forged it.

Ran (DH Kivan): The Rif says that the witnesses say that he asked them to forge this document for him. It seems that he disqualifies only a document against that person and for that sum. It seems that R. Chananel and the Rambam agree. Our texts of the Gemara say 'he asked them 'forge for me.''

Rambam (Hilchos Edus 22:5): If a document was contested, i.e. two said that the bearer of the document asked them to forge this document for him, even if it is validated we never collect with it. It seems to me that if the Edei Chasimah themselves testify that the signatures are theirs, we collect with it.

Ran (DH Kivan): Even if the Edei Chasimah themselves testify that the signatures are theirs, perhaps we do not collect with it unless they remember the loan. Witnesses can err about what is their signature, like we find in Bava Basra. However, the Rambam is not concerned for this.

Radvaz: The Rambam (8:1) taught that one may not testify about his signature unless he remembers the loan.

Kesef Mishneh: The Rambam discusses when we can verify the signatures from other documents. If not, we do not collect through the witnesses' recognition unless they remember the loan (Halachah 5). Alternatively, one might have thought that even if the witnesses remember the loan and recognize their signatures, since they do not remember signing we are concerned for forgery and say that it is a Milveh Al Peh (a loan without a document). Rather, since there is Raglayim l'Davar (supporting evidence) that they signed, the document is Kosher.

Rebuttal (Shach CM 31:4): It does not help if we could verify the signatures from other documents, for we do not validate this document from any other document! Also, since we suspect that the witnesses err (about their signatures), it does not help if they remember the loan! Rather, it seems that the Rambam is not concerned lest witnesses err about their own signatures.

Gra (CM 63:6): The Rambam is not concerned because such expert forgeries are not common.

Mishneh l'Melech: Sefer ha'Terumos (17:1:4) says that the Rambam is Machshir the document because we do not establish the rest of Yisrael to be liars. If so, why does the Rambam say 'it seems to me'? The Gemara (Makos 5b) says so explicitly!

Rosh (3:3): Some say that all of Reuven's documents are Pasul unless the Edei Chasimah testify that they signed. The text of the Gemara suggests otherwise. It says that one does not collect with 'this tainted document.' This implies that witnesses say that Reuven asked them to forge a document for (e.g.) 100 (Zuz) against Ploni, and we disqualify such a document. We do not disqualify all Reuven's documents, only one against Ploni. We assume that he had a Milveh Al Peh, and feared lest Ploni deny it. If the witnesses said 'he asked us to forge a document for him' without mentioning the borrower or the sum, Reuven cannot collect with any document, for we fear that he forged one document.

Poskim

Shulchan Aruch (CM 63:1): If Reuven brings a loan document against Ploni, and two say that Reuven asked them to forge it, even if it was validated we never collect with it. We collect only if the Edei Chasimah testify that they saw the loan, or others testify that they saw the witnesses sign, and if the Edei Chasimah testify that the signatures are theirs we collect with it.

SMA (4): The Shulchan Aruch says 'never' to teach that even if the lender seized the money, we take it back. However, we do not tear the document, lest we clarify that it is a proper document.

Shach (1): The Bach (DH Edim) says that if Reuven has two documents against Ploni for the same sum, he collects one of them. Due to the Safek, he collects from buyers from the document with the latter date. This is unlike Sa'if 2, in which witnesses say that he asked them us to forge a document without mentioning the borrower or the sum. Then, there is no limit to what he sought, so Reuven cannot collect with any document.

Shach (2): The Bach (DH Hai) says that if we saw that Reuven had the document before he asked the witnesses to forge, it is Kosher. We do not disqualify it retroactively. Until Reuven sought to forge, he had Chezkas Kashrus. It seems that this is only if witnesses have a Siman in the document or recognize that it is the same one they saw earlier.

Shach (3): The Tur cites the Ramah who says that recognition of the signatures does not help. The Edei Chasimah must testify that they saw the loan, or others must testify that they saw these witnesses sign. The Beis Yosef, SMA and Bach explain that we can join others who recognize the signatures to the Edei Chasimah who remember the loan. Since we are concerned for forgery, this is unreasonable! We should be concerned that there was a Milveh Al Peh, and after it was paid Reuven seeks a document to make Ploni pay again or to take from buyers! Rather, testimony does not help at all. Even if there was a Kosher document, perhaps he paid and took it back, and this is a forgery! However, if Ploni denies that he borrowed, testimony about the loan or seeing the witnesses sign refutes him. The document does not help at all. The Ramah said 'we collect with it only if...' merely for parallel structure with the Reisha.

Rema: Some argue and say that recognition of the signatures does not help.

SMA (7): The Shulchan Aruch gave three methods of validation. Each is a bigger Chidush than the previous one. The Rema argues only about the last one. Even though the Rambam holds that witnesses who testify about their signatures must remember the loan, if they testified about their signatures and left, we assume that they remembered the loan.

Gra (6 DH Likut): The Mechaber holds that when Edei Chasimah testify about their signatures, the Rambam assumes that they remember the loan and we need not ask them. This is like Chachamim (Kesuvos 20b). Everyone rules like this!

Rema (2): If the witnesses did not say 'he asked us to forge this document', rather, Stam (a document), all Reuven's documents are tainted, and we do not collect with them.

SMA (1): If Reuven has two documents against Ploni, it would seem that he should collect one of them. However, the Poskim should have said so. It seems from the Tur that he does not collect at all.