1) THE REASONING OF REBBI MEIR
OPINIONS: The Gemara (18b) quotes the ruling of Rebbi Meir that even when the witnesses' signatures on a document are not authenticated from an external source, the witnesses are not believed to say that they were coerced to sign falsely even when they say they did so because their lives were in danger. The Gemara explains that the ruling of Rebbi Meir is based on the opinion -- as expressed by Rav Huna in the name of Rav -- that when a borrower admits that he wrote a loan contract, the document is binding and does not need to be verified by witnesses. Moreover, the borrower may not invoke the principle of "ha'Peh she'Asar Hu ha'Peh she'Hitir" and claim that he should be believed that he paid back the loan on the basis that he is the one who validated the document in the first place. The fact that he originally could have said that the Shtar was not valid does not give him more trustworthiness.
How does the opinion of Rav Huna in the name of Rav explain Rebbi Meir's ruling?
(a) RASHI (DH Ein ha'Malveh) explains that the Gemara's discussion of Rebbi Meir's ruling with regard to a borrower refers not only to when the borrower admits that he wrote the loan document, but also to when he told the witnesses to sign the document. When Rebbi Meir states (in the case of the Mishnah) that even when the signatures are not authenticated from an external source the witnesses are not believed to say that they were coerced to sign falsely, he refers to a case in which the borrower admits that he wrote the Shtar himself and had the witnesses sign it. Accordingly, there is no reason to believe the claim of the witnesses (for they are not the "Peh she'Asar").
TOSFOS (DH Ta'ama) questions this explanation. If the focus of the case is the borrower, why do Rebbi Meir and the Rabanan discuss a case in which the focus is the claim of the witnesses?
(b) TOSFOS quotes RABEINU TAM and the RI who explain that the reasoning of Rebbi Meir in the case of the Mishnah is identical to the reasoning in the case of a borrower who is not believed to say that he repaid the loan when he admits that he wrote the Shtar. In both cases, the claimant (the borrower or the witnesses) is not believed on the basis that he could have claimed that the Shtar was forged. (The Rabanan maintain that this logic applies only in the case of a borrower but not in the case of witnesses; see RITVA.)
Why, though, is this claim not valid?
1. Tosfos (DH Modeh) explains that the borrower is afraid to use the claim that the Shtar is forged, because the lender can prove him wrong by bringing proof that the Shtar is legitimate. Accordingly, he has no believability on the basis that he could have claimed that the Shtar was forged. On the other hand, he is not afraid to say that he paid back the loan, since there is no way that the lender can prove that he is lying.
2. Tosfos adds another explanation in the name of Rashi. According to Torah law, a Shtar is considered valid and Beis Din does not need to be concerned that it is forged. The Rabanan decreed that when one claims that the Shtar is forged, the Shtar may not be used until it is verified. Accordingly, when the borrower or witnesses agree that the Shtar was not forged, the Halachah follows the Torah law that the Shtar is valid.
This answer is difficult to understand. Although according to Torah law the Shtar is valid, the borrower still should be able to claim that the Shtar is invalid mid'Rabanan, since the Rabanan decreed that his claim necessitates validation of the Shtar.
The KOVETZ SHI'URIM (volume 2, 3:4) writes that Rashi here has a different understanding of the concept of "Migu." Normally, the concept of "Migu" means that one should be believed with his present claim because had he wanted to lie, he would have said the better claim. Rashi here understands that a "Migu" gives a person believability for a different reason: since the person would be believed if he would make a certain claim, any claim he makes is believed as though he actually made that claim.
The Halachah states that a Shtar is considered to have been authenticated by Beis Din as long as no one actually claims that it is forged. Since the enactment of the Rabanan which permits a person to challenge the validity of a Shtar is a novel concept, the Shtar's validity becomes subject to doubt only when its validity is actually challenged. When no one challenges the Shtar's validity, the Shtar is assumed to be valid. Accordingly, in the case of the Gemara here, there is no "Migu" that the person could have said a better claim, because the better claim (the fact that one may challenge the validity of the Shtar) is a novel concept that it is not taken into account unless it is actually utilized. The Shtar is considered a valid Shtar, and everything written in the Shtar is taken at face value, unless someone actually claims that the Shtar is forged.

19b----------------------------------------19b

2) KEEPING A PAID "SHTAR"
OPINIONS: The Gemara quotes Rebbi Yehoshua ben Levi who rules that a person may not keep a loan document (Shtar) in his possession after the borrower has repaid the loan. Similarly, it was stated in the name of Rav that a person may not keep a Shtar Amanah (or Shtar Pasim; see RASHI to Kesuvos 79a, DH Shtar Pasim) in his possession. A Shtar Amanah is a signed loan document written by a potential borrower, who gives it to his potential lender to hold in the event that he later decides to borrow money. In both cases, if the lender is permitted to keep the Shtar the possibility exists that he will demand money from the borrower unfairly, either willfully or by mistake (he will forget that the borrower does not actually owe him the money).
The Gemara explains that there is a difference between these two opinions. Rebbi Yehoshua ben Levi, who says that a lender may not keep in his possession a paid Shtar, certainly would agree that one may not keep in his possession a Shtar Amanah. Rav, however, who says that one may not keep in his possession a Shtar Amanah, may permit a lender to keep a paid Shtar in his possession. It is possible that the lender did not give back the paid Shtar due to "Peshiti d'Safra." RASHI (DH Peshiti d'Safra) explains that the borrower is the one who must pay the expense of producing the actual loan document. Occasionally, a borrower is so destitute that he cannot afford to pay the scribe who writes the Shtar, and the kindhearted lender loans him the money for that as well. The lender is entitled to hold the Shtar -- even after the primary loan has been repaid -- until the borrower remunerates him for the "Peshiti d'Safra." Hence, Rav might not prohibit the lender from keeping the paid Shtar in his possession.
What does Rebbi Yehoshua ben Levi maintain with regard to "Peshiti d'Safra"? Does he argue that the lender indeed is prohibited from withholding the document even when the borrow did not yet reimburse him for the "Peshiti d'Safra"?
(a) TOSFOS (DH Zimnin) and the ROSH write that Rebbi Yehoshua ben Levi understands that when the lender loaned the money for the writing of the Shtar, he must return the document immediately after the borrower reimburses him. Rebbi Yehoshua ben Levi agrees that the lender does not have to give back the document until he is reimbursed for the "Peshiti d'Safra." This is also the understanding of the RITVA.
(b) The RAN states that Rebbi Yehoshua ben Levi understands that "Peshiti d'Safra" is no reason to withhold a paid Shtar, despite the fact that giving back the paid Shtar compromises the lender's ability to get back the money he loaned for the "Peshiti d'Safra."
The SHACH (CM 57:4) questions why the SHULCHAN ARUCH (CM 57:1) rules like the Ran and ignores the opinions of Tosfos, the Rosh, and the Ritva (which the Shulchan Aruch himself quotes in BEDEK HA'BAYIS on the BEIS YOSEF, CM 57). The Shach adds that it is clear to him that even according to the Ran, one certainly may keep the Shtar in his possession if he tears it or writes on it that the debt has been paid.
The KORBAN NESANEL (2:11:4) asserts that the Shach misunderstood the Rosh. He explains that the Rosh does not disagree with the Ran. Rather, the Ran refers to a case in which the lender neither marks the Shtar as unpaid nor gives a receipt to the borrower for the repayment of the loan. In such a case, the lender may not withhold the Shtar due to "Peshiti d'Safra." When the Rosh rules that the lender may withhold the Shtar, he refers to a case in which the lender writes that it has already been paid or gives the borrower a receipt.
The Korban Nesanel proves that this is the intent of the Rosh (and Tosfos and the Ritva) from the Gemara in Bava Metzia (68a). The Gemara there is concerned about a document akin to a Shtar Amanah ("Shtara d'Mechuzai") falling into the hands of the lender's heirs when the lender meets an untimely death. The heirs would have no reason to suspect that the Shtar is not genuine, and they would proceed to collect the loan as stated in the document. Accordingly, the Rosh cannot mean that a lender may keep the Shtar (until he gets back the "Peshiti d'Safra") without making any note that it was paid, because the same concern that applies in the case in Bava Metzia applies in the case of a paid Shtar. It must be that the Rosh means only that when the lender makes a note on the Shtar that it has been paid, or gives a receipt for the borrower, is he allowed to keep the Shtar.

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