Gemara

(Rav Yosef): If Shimon says 'I willingly had Bi'ah with Levi', he is a Rasha (and cannot testify) - "Al Tesht Rasha Ed".

Rejection (Rava): One is like a relative with respect to himself, so he cannot establish himself to be a Rasha (to disqualify himself from testimony. Therefore, we split his testimony and ignore his admission that he consented, and he joins with a second witness to kill Levi.)

25a: Ploni testified 'I saw Bar Binisus borrow on Ribis', and Almoni testified 'I borrowed from Bar Binisus on Ribis.' Rava disqualified Bar Binisus.

Question: Rava said that one who borrows on Ribis is disqualified. If so, Almoni is a Rasha - "Al Tesht Rasha Ed"!

Answer: Rava ruled like he taught above, that one is a relative with respect to himself. He cannot disqualify himself by saying that he is a Rasha.

Kesuvos 18b (Mishnah): If witnesses say 'these are our signatures, but we were (monetarily) forced to sign', they are not believed if other witnesses recognize their signatures.

Bava Kama 106a (Rav Huna citing 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.

Bava Basra 43a (Beraisa): If Reuven pledged to give a sum for needs of the city, people of the city cannot judge or testify about it.

Question: Why can't two forfeit their shares in it and judge the case?

Rishonim

Rif and Rosh (Bava Kama 9:27): Rav Nachman said that he just explains how Rav must learn. This shows that the Halachah does not follow Rav.

Rambam (Hilchos To'en 2:11): If Levi denied David's claim and swore mid'Oraisa or Heses, and witnesses came and testified that he swore falsely, he must pay and he is established to be suspected to swear falsely.

Rambam (Hilchos Edus 12:2): One cannot disqualify himself for testimony by saying that he transgress. If Shimon testified 'Reuven lent on Ribis', and Levi testified 'I borrowed from Reuven on Ribis', Reuven is disqualified. Even though Levi says that he borrowed on Ribis, he is believed about Reuven, but not about himself. If Shimon says 'Ploni had Bi'ah with me', whether Shimon says that he wanted this or was coerced, he joins with a second witness to kill Ploni.

Question (Kesef Mishneh): In Kesuvos, we do not believe witnesses who say that they signed due to monetary coercion, for one cannot disqualify himself. Why don't we divide the testimony, and believe that they were coerced, but not that the coercion was monetary?

Answer (Kesef Mishneh): There, their primary testimony is that they did not see the loan. Once that say that they signed about something they did not see, they are Resha'im, unless they will explain that they were mortally coerced. Therefore, we cannot divide the testimony. Here, the primary testimony is that Ploni is Chayav Misah.

SMA (CM 34:62): The Rambam is unlike the Mordechai, who says that he must say that the lender did not collect any Ribis.

Rosh (Sanhedrin 3:8 and Tur CM 34:38): R. Yakir says that if Levi (totally denied David's claim and) swore to contradict one witness, the witness and David join to disqualify Levi (for swearing falsely). David is not considered to be partial, for Levi already swore and is exempt from paying him, just like one who borrowed on Ribis is not considered partial and joins to disqualify the lender.

Rosh (Teshuvah 11:2, cited in Beis Yosef CM 34 DH v'Chosav ha'Rosh): The one who swore is not disqualified immediately. He is disqualified from when the witness and his opponent testify, and inwards. Perhaps his opponent claimed falsely, and we disqualify someone based on one witness!

Question (Bedek ha'Bayis): He is disqualified retroactively from when he swore!

Mordechai (693): If Yo'av stole from Moshe, Moshe can join with one witness to disqualify him from testimony and Shevu'ah, for Moshe is no longer partial. Riva applies this also to one who swore to contradict one witness. Even though he exempted himself through his Shevu'ah, he is immediately disqualified from now and onwards for testimony and Shevu'ah. Avi ha'Ezri says that Almoni testified 'I borrowed from Bar Binisus on Ribis.' He did not say that he paid, therefore, he was impartial. R. Yakir learned from here (Sanhedrin 25a) that if two summonsed Levi to Din, whether together or one after the other, and he swore to them, they can join to disqualify him.

Hagahah: The Rif (in Sha'arei Shevuos) disqualifies one who denied a deposit, but not one who denied a loan. It seems that this is because it is normal to make a borrower swear that he does not owe, and perhaps the lender owed him for another reason, and he swore truthfully. It is normal to make a Shomer swear about whether he deposited with him. Since witnesses testify that he did, he swore falsely. This is why the Mishnah disqualified for false oaths about deposits or Shevu'as ha'Edus (denying to know testimony), but not for oaths about loans. My Rebbi says that the Riva disqualifies immediately from when he denied. If he swore after denying and before he swore, it is invalid. This is why he says only for a deposit. I said that he discusses only a deposit, for one is not disqualified for swearing about a loan.

Rebuttal (Rivash 339): We are forced to say that we divide Ploni's words, and believe that Bar Binisus lent on Ribis, but not that he lent to Ploni on Ribis. What one says about himself is not testimony at all. If one pledged to give a sum for needs of the city, people of the city cannot judge or testify about it. We asked, why can't two forfeit their shares and judge it? Even though a judge must be Kosher from the beginning to the end, and before forfeiting his share he was partial, since he was a party to the case, he is not considered a Pasul witness at all. He becomes a witness only when he forfeits his share, so he was Kosher from the beginning. The Rashba says that if Ploni stole from Levi, Levi can join to disqualify him only before claiming the theft from him in Beis Din. He cannot do so after Ploni swore to deny it, for the Torah believed his oath. The same applies to one who swore to deny stealing from two partners. We cannot say that the Torah believed him to swear, but after he swore, they disqualify him! Further, if David brought a witness that Ploni stole from him, and Yosef did similarly, and Ploni swore against each witness, the witnesses cannot join to disqualify him.

Bach (CM 34:38): R. Yakir disqualifies immediately from when he swore, for Beis Din knows from the witness and his opponent that he swore falsely. The Agudah cites the Riva, but does not say that he is disqualified immediately, rather, after they testify about him, like the Rosh says. The Mordechai must mean that once they testify about him, he is disqualified retroactively from when he swore. This is Bedek ha'Bayis' intent.

Poskim

Shulchan Aruch (CM 34:26): If Reuven testified 'Shimon lent to me with Ribis', and there is a witness with him, they join to disqualify Shimon. Even though Reuven admits that he himself is a Rasha, we divide his words and believe him about the lender, but not about himself.

Shach (28): We divide one's words (and believe him about part) only when we do not suspect that he is partial. If not, we are concerned lest he testifies for his own benefit. A sharecropper cannot testify when there are crops in the field. We do not divide his words (to believe him only about Chazakah, but not to receive his share of the crop (Bava Basra 46b). The Gemara suggested that two can pardon their share and judge the case. Without pardon, even though they are not Pasul witnesses, they would be partial. The Ramban and all Poskim explained it differently.

Rema: Likewise, a victim of theft can testify about the thief, like a borrower can testify about the lender, as long as he does not benefit from his testimony.

Gra (55): Kidushin 43b proves that a witness may not benefit from his testimony.