Gemara

Sanhedrin 10a (Rava): If Shimon says 'Levi had Bi'ah with my wife', Shimon can join with a second witness to kill Levi, but not to kill his wife. (We split his testimony, and believe it regarding Levi, but not regarding Shimon's wife.)

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 according to another of his teachings, that one is considered a relative with respect to himself, so he cannot disqualify himself by saying that he is a Rasha.

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.

(Rava): Presumably, Rav's law applies only to a loan, for a borrower acquires the money. One can say that the oath is in place of repayment;

A deposit always belongs to the owner, Reuven cannot acquire it through an oath! However, Rav said his law even regarding a deposit, for the verse he expounds discusses a deposit!

Kesuvos 87a (Mishnah): A woman must swear (mid'Rabanan) to receive her Kesuvah if one witness says that it was paid.

88a (Rav Papa): If he is clever, he will force her to take an oath mid'Oraisa. He pays the Kesuvah again in front of a different witness. He joins the two witnesses, and claims that the first payment was a loan.

Rishonim

Rif and Rosh (Bava Kama 9:27): Rav Nachman said that he just explains how Rav must learn, but he himself disagrees. 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.

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 until they (the witness and his opponent) testify. 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): R. Yakir learns from 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.

Teshuvas Rashba (attributed to Ramban, 109): The Ramah says that one claimant can join with a witness to disqualify the Nidon (for swearing falsely), but two claimants cannot join. I say that if there were a distinction between one and two, the Gemara would have specified. Letter of the law a claimant joins to disqualify him, even for capital cases (e.g. one who testified 'Ploni had Bi'ah with my wife'), therefore, even two claimants can disqualify him. I hold that even one claimant cannot join a witness to disqualify the Nidon, for if so, anyone who swore to contradict one witness would be disqualified! Likewise, two who claimed from one would disqualify him. The Torah believed one to swear to deny a claim in these cases! Do we believe the Nidon regarding money, and disqualify him for testimony?! Also, if one made Ploni swear once, it would be forbidden to make him swear again! The Ra'avad (on Rif Kesuvos 48a) explained (Kesuvos 88a) 'if he is clever, he will force her to take an oath mid'Oraisa', after she swore to him. The Ri mi'Gash says that one is disqualified only through witnesses. Normally, one does not swear if he is in doubt; Chachamim were concerned only about one who thinks that another owes him an old loan. It seems that the Torah believed one who swore. If each of two people had one witness for claims against Ploni, and he swore to contradict them, those witnesses cannot disqualify Ploni. If not, a person could not swear to contradict one witness more than once! In the case of Ben Benisus, he had not yet sworn when the borrower joined to disqualify him.

Terumas ha'Deshen (2:236): If one swore (falsely) to keep his money, he is not disqualified, for perhaps he was stalling until he will be able to pay. One who swore (falsely) to collect money is disqualified, like one who swore (falsely) about a deposit.

Yam Shel Shlomo (Bava Kama 9 47): In Siman 87, the Tur retracted from his ruling (in Siman 34) like R. Yakir, and said that the witness joins with someone else, but not with the first claimant. I agree with the Rashba, except for one matter. If Ploni swore against one witness, and then David claimed from him with one witness, after Ploni swears the second time the witnesses can join. They both testify that Ploni swore falsely; they do not testify about money, for neither knows about the money that the other knows about. The claimants cannot even join to disqualify him. After he swore twice against one witness (a different witness each time), he cannot swear again. We do not distinguish whether he swore a false oath mid'Oraisa or mid'Rabanan, whether it was to avoid paying or to collect. However, if he swore to pay by a certain date and did not, and he claims that he was Ones and was unable to pay, if we cannot prove that he had Metaltelim or Kelim to sell, he is not disqualified from testimony and swearing. The same applies if he said that he forgot.

Yam Shel Shlomo (ibid.): We do not find R. Yakir's law in earlier Poskim. The Rosh wrote so in a Teshuvah, but not in his Pesakim. Once he swore, he is exempt from the claimant. Why can the claimant join with one witness to disqualify him?

NOTE: I did not see this explicit in Teshuvas ha'Rosh. There, he discusses disqualification through the claimant and one witness!

Poskim

Shulchan Aruch (CM 87:29): If Levi totally denied David's claim and one witness contradicted Levi and Levi swore to contradict the witness, and afterwards David brought another witness, he joins with the first. Levi pays, even though he swore. He became suspected to swear falsely through two witnesses.

Beis Yosef (DH u'Mah she'Chosav): Obviously, after the second witness comes, Levi is suspected to swear falsely! However, why did the Tur write that David and the first witness do not join to disqualify Levi? If they would, there would be no need to write that he becomes suspected through the two witnesses! This is unlike what the Tur wrote in Siman 34 in the name of R. Yakir (and did not say that he disagrees). We cannot distinguish between disqualifying from testimony and being suspected to swear falsely, for anyone disqualified from testimony due to an Aveirah is suspected!

SMA (89 and Prishah 44): Here, in Siman 87, the Tur discusses when there are two witnesses. Levi is disqualified automatically. In Siman 34 the Tur discusses when David and the witness did not testify to disqualify Levi. He is not disqualified automatically, like the Rosh says.