Gemara

30b (Beraisa - R. Meir): If a loan document specifies that Ribis (usury) will be charged, we fine the lender, and he may not collect even the principal;

Chachamim say, he collects the principal, but not the Ribis.

73a (Beraisa): If witnesses testified that Levi stole and slaughtered, and they were Huzmu about (others testified that they were not there to witnesses) the slaughter, Levi pays double, and the witnesses pay triple;

R. Yosi says, this applies to two testimonies. Regarding one testimony, partially nullified testimony is totally nullified (Levi is exempt).

'Two testimonies' means like two testimonies, one after the other. One testimony is when they testified about the theft and slaughter together.

Assumption: Both Tana'im hold that anything done Toch Kedei Dibur is k'Dibur (a continuation of the previous matter). They argue about whether or not Zomemim witnesses are disqualified retroactively from when they testified falsely.

Rejection: No, they argue about whether or not Toch Kedei Dibur is k'Dibur. Chachamim say that it is not k'Dibur. R. Yosi says that it is.

Rishonim

Rambam (Hilchos Malveh 4:6): If a loan document specifies that there is Ribis mid'Oraisa or mid'Rabanan, he may collect only the principal.

Magid Mishneh: The Ramban says that the Ribis is not explicit in the document. The witnesses were unaware, or were mortally coerced to sign.

Rosh (3:8): Why aren't the witnesses Pesulim! We discuss Ribis mid'Rabanan. The witnesses are not disqualified, for they do not profit from it. Or, it can even be Ribis mid'Oraisa, for people think that only the lender and borrower transgress.

Poskim

Shulchan Aruch (CM 52:1): If a document specifies that there is Ribis, one may not collect the Ribis, but he collects the principal from Meshubadim.

Shach (1): Gedolei Terumah says that if Ribis mid'Rabanan was included with the principal, all agree that one collects the principal from Meshubadim. This is even when the witnesses knew, for if not the Ramban agrees even about Ribis mid'Oraisa.

Ketzos ha'Choshen (1 DH uv'Ikar): Tosfos (Chulin 14a DH ha'Shochet) explains that an animal slaughtered on Shabbos is Kosher because the Shochet does not become a Mumar through one slaughter. The Ran says that he becomes a Mumar only after the Shechitah. The same applies here. Witnesses do not transgress for a Milveh Al Peh, for perhaps it will not be collected. They transgress only after signing.

Maharit (138, cited in Ketzos ha'Choshen 1 DH u'Motzasi): If there was a Cherem not to be a witness on a Kidushin unless a Chacham who teaches was there, R. Shmuel Di Mudina (EH 21) disqualifies testimony of witnesses who transgressed this. The Shach leaned to agree, but said that Rabanan do not invalidate such Kidushin; perhaps the witnesses are disqualified only after the Kidushin. If so, what was Tosfos' question? He answered that regarding Shechitah, they transgress Shabbos from the beginning of Shechitah for Netilas Neshamah (killing).

Rebuttal (Maharit): We rule like Abaye, who disqualifies an Ed Zomem retroactively. We disqualify even his testimony Toch Kedei Dibur before his false testimony, all the more so the false testimony itself! Also, witnesses on Kidushin must be Kosher from the beginning to the end. They must testify now, and now they are Pesulim! His answer about Shechitah is improper. The beginning of Netilas Neshamah is destructive. One is liable for it only from when it is useful, i.e. after finishing the Shechitah. Kidushin is different. Witnesses transgress only if they were asked to be witnesses, but not if they merely happened to witnesses Kidushin. Even if they say that they were asked, we split the testimony and believe only that they saw Kidushin.

Defense (of Shach - Ketzos ha'Choshen DH Emnam): Chamas (theft) disqualifies a witness, due to concern lest he lie to gain. For other Aveiros, he is disqualified only if he is liable to be lashed. The Nimukei Yosef (Sanhedrin 5b) disqualifies one who took a vain oath from a Gezeras ha'Kasuv, for he is called a Rasha. He is Kosher for testimony for which one witness, even a woman, is Kosher; we do not suspect that he will lie. R. Yerucham says that if one signed and then transgressed Chamas, we require that his signature was established in Beis Din before he transgressed, lest after transgressing he signed improperly. We do not require this for one who became a relative (e.g. a son-in-law) after signing. A Gezeras ha'Kasuv disqualifies him; he is not suspected to transgress. Tosfos was unsure about Aveiros other than Chamas, for one suspected of one Aveirah is not suspected of others. Therefore, if one became a Rasha, his testimony before he became a Rasha is Kosher. What he testified while transgressing is Kosher, for he is not suspected to lie. This is like the Ran wrote about a Mumar's Shechitah. An Ed Zomem or one who transgressed Chamas is suspected to lie. He is disqualified from when we suspect him. One who transgressed the Cherem is not suspected to lie.

Rebuttal (Nesivos ha'Mishpat 1): If one slaughtered and immediately afterwards ate Neveilah, he is not suspected retroactively about the Shechitah. Testimony is different, for partially disqualified testimony is totally disqualified. Even if they testified two testimonies for two people and they were relatives to one, the Rif (Makos 3a) brings two opinions about whether both are Batel. Here, the testimony about Shechitah is disqualified through Hazamah, so all agree that the entire testimony is Batel. The Maharit is correct. One is not a Mumar until he is established, but surely he is disqualified from testimony. If witnesses signed a Get on Shabbos she is not divorced, for they are disqualified mid'Oraisa.

Meshovev Nesivos (DH v'Im): The proof from a Get is invalid. A witness must sign, in addition to his name, his father's name or the word 'Ed'. Once he wrote two letters he is disqualified, so the end of his signature is invalid. (Hagahah - Maharik 109 says that even though he is not disqualified after one Melachah, he is suspected.)

Note: What is the source to say that a signature requires two letters? Some Amora'im signed a single letter or drew a picture in place of a signature (Gitin 36a). The Chasam Sofer (87b DH Tziyer) says that a picture suffices without 'Ed'. If the word 'Ben' was already written in the Get, one can sign his name and his father's in two letters! Chinese has no letters; every word or name is a separate character. Chachamim exempt one who wrote a letter with a Siman (e.g. period) to show that it represents a whole word (Shabbos 104b). However, it seems that Nesivos ha'Mishpat discusses standard signatures.

Meshovev Nesivos (DH d'Hasam): The proof from the Rif is invalid. Tana'im argue about whether or not the witness is disqualified retroactively. According to Nesivos ha'Mishpat, in either case the first testimony is Batel! Rather, if they are disqualified from now and onwards, the first testimony is Kosher. The Rif discusses one testimony. Certainly a Pesul due to an action does not disqualify a different action done just before it!