Gemara

(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.

Rishonim

Rif (Bava Metzia 42b): The Halachah follows Chachamim, who do not fine Heter due to Isur.

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 witnesses are not disqualified, for not everyone knows that they transgress a Lav. Some say that he collects Bnei Chorin (but not from Meshubadim, i.e. property that the borrower sold) when the borrower admits or other witnesses testify about the loan, but the document is Pasul. The Ramban says that Ribis is not explicit in the document, and the witnesses did not know about it, or they were mortally coerced to sign. If they knowingly and willingly signed for Ribis, they are disqualified. This is primary.

Mishneh l'Melech (6): Sefer ha'Chinuch (74, or 68 in modern editions) says that since the lender is not lashed, since he can return the Ribis, we do not say that the borrower or witnesses are lashed. Also the Kesef Mishneh (Edus 10:4) suggests that they are not disqualified because they are not lashed.

Rosh (Bava Kama 3:8): Why do Chachamim allow collecting principal from Meshubadim? Also, why does R. Meir disqualify only due to a fine? The witnesses are Pesulim! The Halachah follows Abaye, that one who eats Neveilos to anger Hash-m is Pasul for testimony! The Mishnah (Sanhedrin 24b) did not explicitly list witnesses on a loan with Ribis among people disqualified from testimony. However, it did not explicitly list the borrower, either. The Gemara said that 'one who lends with Ribis' means 'a loan with Ribis', and includes him. Likewise, it includes the witnesses! Do not say that since the document specifies Ribis, Beis Din will not collect it so they do not transgress. Witnesses transgress from the time Ribis is contracted (Bava Metzia 75b, Bava Basra 94b)! Rather, we discuss Ribis mid'Rabanan. The witnesses are not disqualified, for they do not profit from it. Perhaps it can even be Ribis mid'Oraisa, for people think that only the lender and borrower transgress. Ge'onim do not list the witnesses and Arev (guarantor) among Pesulei Edus for this reason.

Hagahos Ashri: The Ribis is explicit in the document. If it said only 'Ploni owes 100', he would not collect anything, lest he collect the Ribis. This is like a predated document; it is totally Pasul. Even though we collect principal when the Ribis is evident, one who finds the document must tear it. If it comes to Beis Din, they tear it, for the owner transgresses through its existence.

Beis Yosef (YD 161 DH v'Da): A Tosefta (Bava Metzia 5:9) discusses finding such a document. Hagahos Ashri added the last words (the owner is Oved, i.e. loses). Normally, the lender is called the owner. If we tear the document, he loses the ability to collect the principal! If the borrower is called the owner, what does he lose? He owes principal! Rather, it has Ribis mid'Rabanan; perhaps it will be collected by mistake. Alternatively, the lender owns it. We are concerned a Beis Din will rule like R. Meir, and he will not collect at all. It is better that he collect through witnesses who saw the document. They are like a document, and enable collecting from Meshubadim.

Bach (Sof ha'Siman): The correct text is 'the owner is Over (transgresses).'

Poskim

Shulchan Aruch (YD 161:11): If there is Ribis mid'Oraisa or mid'Rabanan in a document, one may collect only the principal. This is if the Ribis is evident.

Rema: This is when the document says how much is the principal itself, or it mentions only principal and witnesses testify about the Ribis. Whoever gets the document should tear it, lest it be used to collect Ribis.

Shach (18): 'One tears it' refers to the Reisha, when the Ribis is explicit. Obviously, if it mentions only principal, we do not tear it.

Shulchan Aruch (ibid.): If the Ribis is not evident, but is included with the Ribis, he does not collect even principal.

Beis Yosef (DH ba'Meh): Rashi and Tosfos say that the Ribis is evident. If not, even Chachamim decree lest he collect the Ribis. The Ramban says that the Ribis is not evident. If we need the witnesses to validate the document, they are believed to say that they were unaware of the Ribis or were mortally coerced to sign. Talmidei ha'Rashba explain that if the Ribis were evident, they signed Isur; they would need a proof to be Machshir themselves. Why did the Tur rules here like the Ramban, and in Choshen Mishpat like Tosfos?

Darchei Moshe (6): The Mordechai (Bava Metzia 440,456) holds that if the Ribis is not evident, he does not collect at all, even if the borrower admits. This is a fine. It seems that the Tur agrees. The Maharik (17) says that he collects principal from Bnei Chorin.

Shach (20): The borrower need not pay anything. However, it is proper that he give from his profit to poor of the city.

Taz (7): If the Ribis is not evident, the witnesses are disqualified.

Taz (8): The case is, the document mentions only the debt, and other witnesses testify that part of it is Ribis. This is like the Ramban.

Rebuttal (Nekudas ha'Kesef): The Shulchan Aruch is like Tosfos and most Meforshim, that the witnesses are Kosher because they do not know that they transgress. This is explicit in CM 34 and 52.

Beis Yosef (DH Kosav ha'Rivash): A case occurred in which a man promised to give a dowry of 400 at the time of Nisu'in, then agreed with his son-in-law that instead e will give 400 at the end of four years, and 80 Ribis each year until then. They wrote this obligation in the document. This was a mistake. Before Nisu'in, one may add to the dowry like this. However, this document connotes that it is Ribis mid'Oraisa, and Beis Din will collect only the principal. They should have written as if it is all payment of principal.

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.

Prishah (1): Perhaps the Tur mentioned that one does not collect the Ribis to show why the witnesses are not disqualified, i.e. they knew that the lender will not be able to collect the Ribis since it is explicit in the document.

Shach (1): Gedolei Terumah (46:2:3) says that since Tosfos was unsure whether witnesses who knew that there is Ribis are disqualified, and the Ramban was sure, we follow the Ramban. Tosfos was unsure only in Bava Metzia, but in Bava Kama and Bava Basra he was sure that they are Kosher, and the Rosh, Mordechai, Tur, Hagahos Ashri and other Poskim agree! Rather, it is a Safek.