1) ACCEPTING TESTIMONY OF A PERSON WHO CAN HEAR BUT CANNOT SPEAK
OPINIONS: The Gemara discusses various cases of a person who saw an event but then became disqualified from testifying, such as a person who became mute. Does this imply that a person who can hear but cannot speak is disqualified from testifying?
(a) The RASHBAM (DH v'Nischaresh) maintains that a person who can hear but cannot speak is not intrinsically disqualified from being a witness. Rather, a technical problem prevents him from testifying. His only method of testifying would be through writing, and since written testimony is not valid, it is not possible for him to testify.
(b) The ROSH (8:24) writes that such a person is disqualified from testimony because the verse states with regard to witnesses, "Im Lo Yagid" -- "if he does not say [the testimony]" (Vayikra 5:1), which implies a requirement that the witness be able to verbally express the testimony.
The KETZOS HA'CHOSHEN (46:19) writes that one who cannot speak may not serve as a witness to sign a Shtar, even though no verbal testimony is necessary. Such a person is disqualified from all forms of testimony. This is consistent with the opinion of the Rosh. The Rashbam, however, would permit the signature of such a person on a Shtar, as the DIVREI MISHPAT points out. The Rashbam maintains that a person who cannot speak is essentially a valid witness but he cannot testify in a normal case because his written testimony is not accepted; however, in a case where written testimony is valid (such as a Shtar), he is a valid witness.
RAV REUVEN GROZOFSKY (Yevamos 28:3) suggests that it is possible that even the Rosh would accept the a mute person's signature on a Shtar. He explains that there are two explanations for why written testimony is not accepted. The first explanation is that writing does not have the legal status of speaking. The second explanation is that although writing does have the legal status of speaking (and thus when the witness signs it is as if he is speaking), nevertheless written testimony cannot be accepted because the court must hear testimony from the witness and not just be aware of testimony that took place at a different time (such as when the witness signed the Shtar). If the second approach is correct, then the reason why a Shtar is acceptable in court is that it is considered to be testimony that was already accepted and verified by Beis Din at the time that it was signed. Consequently, even though a person who is mute is disqualified from testifying in person because he cannot convey his testimony verbally, his signature on a Shtar is accepted. His signature attesting to what is written in the Shtar is considered to be testimony through speech, and such testimony suffices in the case of a Shtar. (Y. MARCUS)
2) WITNESSES WHO CONTRADICT A PERSON FOR HIS BENEFIT
QUESTION: Rebbi Aba sent to Rav Yosef his ruling in a case in which a lender demands the repayment of a loan from the borrower, and he presents a Shtar that attests to the loan. The borrower claims that he already paid back half of the loan. Witnesses later come and testify that the borrower paid back the entire loan.
Rebbi Aba (according to the explanation of most Rishonim) ruled that the borrower must make a Shevu'ah of Modeh b'Miktzas in order to exempt himself from paying the half of the loan that he denies that he owes. The other part of the loan (which the borrower admits that he owes, but which the witnesses say that he paid back) may be collected by the lender, but only from Nechasim Benei Chorin and not from Nechasim Meshubadim (land which the borrower owned at the time that he took the loan and which he later sold). Since the witnesses say that the borrower already paid back the entire loan, the buyers (Lekuchos) who bought his land can claim that they are relying on the testimony of the witnesses.
This ruling is difficult to understand. If the testimony of the witnesses is reliable such that the lender may not collect the loan from Nechasim Meshubadim, then why does the borrower need to make a Shevu'ah on the part of the loan that he says he already paid? Whenever a person is Modeh b'Miktzas, and he admits that he owes part of the debt while he claims that he paid back the other part, and he has witnesses who support his claim that he paid back the other part, he does not have to make a Shevu'ah! If, on the other hand, the testimony of the witnesses is disregarded because the borrower himself contradicts them (and the borrower's word is accepted since he is believed, with regard to obligating himself, to contradict the testimony of witnesses), why, is he believed with a Shevu'ah to say that he paid back half of the loan? The lender is holding a valid Shtar that says that he owes the entire loan (and the Shevu'ah of Modeh b'Miktzas applies only when there is no Shtar)! (RA'AVAD cited by RASHBA, RITVA)
(a) The RA'AVAD answers that the case of the Gemara is when the borrower states only, "I paid back half," and he does not say, "I paid back half and not more" (see, however, RASHBAM in DH v'Loveh Amar). Since he says nothing about the other half of the loan, it could be that he means that he does not remember whether he paid it back or not. Perhaps he is saying that he is certain that he paid back half of the loan, but he does not remember whether he paid back the other half. The witnesses come and say that they remember that he paid back all of the loan. Accordingly, the borrower is not contradicting the witnesses (and thus he should be exempt from paying anything, even without a Shevu'ah).
However, perhaps this is not his intention when he says that he paid back half, but rather he means that he only paid back half and not more.
Since there is a doubt whether he is in agreement with the witnesses or whether he is contradicting them, Beis Din rules stringently with regard to himself and say that he is contradicting the witnesses, and thus their testimony is disregarded. However, Beis Din accepts their testimony with regard to weakening the strength of the Shtar against him, and thus Beis Din treats this case like a normal case of Modeh b'Miktzas and requires the borrower to make a Shevu'ah in order to exempt himself from paying half.
On the other hand, with regard to the people who bought his land, Beis Din assumes that he is not contradicting the witnesses and Beis Din accepts their testimony. Therefore, the lender cannot collect from Nechasim Meshubadim.
(b) The RAMAH and RABEINU YONAH explain that, in essence, Beis Din accepts the testimony of the witnesses (and therefore the lender cannot collect from the Nechasim Meshubadim from the Lekuchos). Even though, according to the borrower's words, the witnesses are not telling the truth (and thus Beis Din should not exempt his word at all since there is a Shtar against him), Beis Din accepts his claim with a Shevu'ah because he has a Migu that he could have said that he paid back everything. He would have been believed because there are witnesses who support that claim. (The Migu does not exempt him from the Shevu'ah, though, because of the general rule that a Migu cannot exempt a person from a Shevu'ah.)
(c) Others (see ME'IRI) explain that the testimony of the witnesses (who say that he paid) is able to nullify the power of the Shtar (which says that he did not pay). However, the testimony of the borrower himself cancels the testimony of the witnesses. Consequently, the claim against him (that he owes the entire amount) is upheld only because of his own admission. Therefore, he is believed with a Shevu'ah of Modeh b'Miktzas to exempt himself from the half that he denies, while his word is not accepted to obligate the Lekuchos.