WHY THE MISHNAH DID NOT TEACH A DIFFERENT CASE
Question: The Mishnah should teach that R. Yehoshua agrees that if Reuven tells Shimon 'I borrowed 100 from you and repaid you', he is believed!
Answer: The Tana wanted to teach in the Seifa that if there are witnesses about the matter Reuven admitted to, he is not believed. Regarding the suggested case, this is not true!
We hold that if one borrowed money with witnesses (without a document), he may return the money without witnesses (so Reuven would be believed!)
Question: The Mishnah should teach that R. Yehoshua agrees that if Reuven tells Shimon 'I borrowed 100 from your father and repaid half', he is believed (without an oath)!
Answer: We cannot teach this according to either Tana (in the coming Beraisa)!
Chachamim say that Reuven is like a Meshiv Aveidah (one who returns a lost object. Rashi - he need not swear when Shimon claims from him. Tosfos - he need not swear even if witnesses knew about the loan!)
R. Eliezer ben Yakov holds that Reuven must swear!
(Beraisa - R. Eliezer ben Yakov): Sometimes a person must swear due to his own claim, e.g. if Reuven told Shimon 'I borrowed 100 from your father and repaid half';
Chachamim say that he is like a Meshiv Aveidah, and he need not swear.
THE OATH FOR PARTIAL ADMISSION
Question: Doesn't R. Eliezer ben Yakov agree that a Meshiv Aveidah need not swear?
Answer (Rav): (He agrees when there is no claim against him.) The case is, a child claims that he owes the full 100.
Question: We learned that one does not swear in response to the claim of a child, deaf person or lunatic!
Answer #1: Rav meant that an adult claims the full 100. He calls him a child, because an adult is like a child regarding his father's affairs.
Question: If so, why is this called swearing due to his own claim? It is someone else's claim!
Answer: It is someone else's claim, and Reuven's own admission.
Objection: Every case (of the oath for partial admission) is due to someone else's claim and a person's own admission!
Answer #2 (to Question (c)): (Really, a child claims. One does not swear due to a child's claim of his own money, but one swears due to a child's claim of his father's money.) The argument is based on Rabah's teaching:
Question (Rabah): Why did the Torah impose an oath for a partial admission? (The person should be believed, Migo (since) he could have denied the entire claim!)
Answer (Rabah): We have a Chazakah (a rule of human nature) that a person lacks the audacity to deny the claim of his creditor.
He would like to deny it entirely, but he cannot be so brazen.
To avoid being so brazen, he would like to admit to the full claim. He denies part, in order to evade the rest of his debt until he can pay it.
The Torah imposes an oath on him, so that he will admit to the full claim.
R. Eliezer ben Yakov says that a borrower could not be brazen to the lender, and not to the lender's son. Therefore, he is not like a Meshiv Aveidah, and he must swear against the son's claim;
Chachamim say that he could be brazen to the lender's son. Since he chooses not to be brazen and to admit, he is like a Meshiv Aveidah, so he need not swear.
WITNESSES WHO SAY THAT THEY SIGNED IMPROPERLY
(Mishnah): If witnesses say 'these are our signatures, but we were forced to sign', or 'at the time we were minors or invalid witnesses', they are believed;
If other witnesses recognize their signatures, or if we can match their signatures to an authenticated document that they signed, they are not believed.
(Gemara - Rami bar Chama): They are not believed only when they claim that they signed due to a monetary threat. If they say that they signed to save their lives, they are believed.
Objection (Rava): No! Once they have testified (by signing), they cannot retract their testimony!
Suggestion: Perhaps this principal refers only to oral testimony, but not to written testimony.
Rejection: Reish Lakish taught that witnesses signed on a document are treated like testimony that was investigated (and accepted) by Beis Din.
Correction: Rami bar Chama's law applies to the Reisha. They are believed only when they claim that they signed because of a mortal threat. If they say that they signed to save their money, they are not believed.
This is because a person cannot establish himself to be a Rasha (to invalidate his testimony).
(Beraisa - R. Meir): (If witnesses say 'these are our signatures, but we were forced to sign',) they are not believed to invalidate the document;
Chachamim say, they are believed.
Question: We understand the Chachamim. Ha'Peh she'Asar Hu ha'Peh she'Hitir (the one who forbade is believed to permit. Since we cannot verify the document without them, they are believed to invalidate it);
But what is the reason of R. Meir?
We understand why they are not believed to say that they were invalid witnesses. Surely, the lender checked that his witnesses are valid!
We also understand why they are not believed to say that they were children. Reish Lakish taught that there is a Chazakah that witnesses sign a document only if all the parties are adults!