Gemara

(Abaye): Wherever the Torah said not to do something, if one transgressed, he accomplished something. If not, he would not be punished!

(Rava): Iy Avid Lo Mehani (his Aveirah does not accomplish anything). He is punished for acting contrary to Torah!

6a (Rav Ashi): Rava and Abaye argue about whether Beis Din forces the lender to return Ribis Ketzutzah (contracted interest):

(R. Elazar): Beis Din forces the lender to return Ribis Ketzutzah.

(R. Yochanan): Beis Din does not force him.

Objection (Rav Acha): That argument is not based on Iy Avid Mehani, rather, on verses!

(R. Yitzchak): R. Yochanan learns from "va'Chai Lo Yichyeh." He will die (at the hands of Heaven); it will not help to return the Ribis.

(Rav Ada bar Ahavah): He learns from "... v'Yaresa me'Elokecha" - one returns usury because he fears Hash-m, but Beis Din does not force him.

(Rava): He learns from the end of the first verse, "(Lo Yichyeh) Mos Yumas Damav Bo" - those who lend on usury are compared to murderers. Just like one cannot undo murder, one cannot undo Ribis by returning it.

(Rav Nachman bar Yitzchak): R. Elazar learns from "va'Chai Achicha Imach" - return it, in order that the borrower will live.

Rishonim

Rambam (Hilchos Sanhedrin 23:1): One may not take Shochad (a bribe) even with intent to judge properly. If one took it, he transgressed, and he must return it when the giver demands it.

Question: Leah borrowed from Shimon. When it was due and he claimed from her, she said that she does not have what to pay. Shimon claimed that Beis Din should collect for her a gift document that her husband wrote to her, through Shibud d'R. Noson (Ploni's creditor may collect directly from one who owes Ploni). The document said that the witnesses warned the husband that she can collect it whenever she wants. She says that she does not want now. Do we honor Shimon's claim?

Rosh (Teshuvah 39:6): Shibud d'R. Noson does not apply here. It is only when someone else owes the borrower. Here, if it was a proper gift document, she already acquired the property. Even though she let her husband eats the Peros, this is her will. When she wants, she can take the land and eat the Peros herself. Do not say that she does not acquire until she wants. He gave Stam (immediately), just the witnesses warned the husband that this is like giving to someone else, and even so he gave to her.

Mishneh l'Melech (Hilchos Malveh 2:6): Why did the Rosh need to say that the document itself did not say that she acquires when she wants, and merely the witnesses said so? Even if the document explicitly stipulated this, Beis Din should force her to claim in order to pay, and if she does not, we collect for the lender from the borrower, like the Rosh wrote in another Teshuvah!

Rashi (Bava Metzi'a 61b DH Ad): Beis Din forces the lender to return Ribis Ketzutzah if the borrower claimed it in his lifetime.

Poskim

Shulchan Aruch (CM 9:1): One may not take Shochad even with intent to judge properly. If one took it, he transgressed, and he must return it when the giver demands it.

Bach (2): This is like the Rambam. This connotes that if the giver does not demand it, the judge need not return it, even if he took it to tilt the verdict. If the giver does not demand it, he pardons it, just like one can pardon theft. The Rambam (Hilchos Malveh 4:13) says that Ribis that one can force the lender to return, the borrower can pardon it. Ribis and Shochad are the same. One can force the taker to return it, but if he does not claim it, it is pardoned.

Bach (DH v'Ika): What is the Rambam's source that the judge must return Shochad? We cannot learn from Ribis, for there the Torah said "v'Chei Achicha Imach" - return to him so that he will live. Perhaps the Rambam holds that the verse is not needed for Ribis. Obviously, since the Torah forbids it, if one transgressed and gave it, it is theft and it must be returned; Iy Avid Lo Mehani. However, one might have expounded that it is not returned, from "Lo Yichyeh" or "v'Yaresa me'Elokecha", like R. Yochanan holds. Therefore, we need v'Chai Achicha Imach. We do not need a verse for Shochad. Since the Torah said not to give, if he transgressed and gave, he must return it, for it is theft.

SMA (3): If the giver does not demand it, the judge need not return it, since the giver gave it willingly, in order to judge properly. This is unlike Ribis, about which it says v'Chei Achicha Imach. It must be returned so that the borrower will live (even if he does not claim it).

Taz: The SMA overlooked Rashi, who says that Beis Din forces him to return it if the borrower claimed it in his lifetime. A case occurred in which Ploni had nothing except for Ribis that he paid to the lender and did not get it back yet. Ploni's creditor wanted to take it due to Shibud d'R. Noson. I say that he cannot. Since the borrower is not claiming the Ribis (either this is the case, or we assume so, for he knows that he will not keep it - PF), there is no money (for Ploni to claim).

Gra (2): The Gemara concluded that Abaye and Rava argue about Ribis Ketzutzah. Just like it must be returned, also Shochad.

Ketzos ha'Choshen (1): Paying a creditor is a Mitzvah, so we force one to fulfill the Mitzvah (Kesuvos 86a). Similarly, here that he has no money to pay his debt, we force him to say that he wants to claim back the Ribis, so he will be able to pay. The Mishneh l'Melech said similarly. In Bava Basra (124a), we say that if a document came against the orphans, the Bechor pays a double share. He may say 'I will not take a double share of the estate, and I will not pay a double share.' Tosfos (DH v'Im) says that this affects a document against the Bechor himself. If the creditor cannot collect the entire loan from one share, he loses the rest. Why don't we force him to claim his double share, in order to pay his debt? There, once he said 'I will not take', he forfeited his Bechorah, for it is called a gift. He cannot retract afterwards (CM 278:10). Here, even if he said that he does not want to claim the Ribis or Shochad, he can claim it whenever he wants. However, since he is forced, this is not Ratzon (desire). This is how I explain why we do not take security for one who must bring a Chatas. Since it requires Ratzon for Kaparah, we cannot force this. I later found that Keneses ha'Gedolah (YD 161) says that we force him to claim the Ribis in order to pay his debt.

Ketzos ha'Choshen: If he pardoned the Ribis or Shochad, he has no further claim to it. Shibud d'R. Noson does not prevent him from pardoning, since there was no claim (so the judge or one who lent on Ribis did not owe yet), like the Bechor who did not want to take his extra share. The Rambam (Hilchos Malveh 4:13) says that pardon helps for Ribis. This is not considered Davar she'Lo Ba l'Olam, since he can claim it whenever he wants. However, the Ge'onim hold that he cannot pardon it, since Ribis is always with the borrower's consent, and even so the Torah forbids it. If he claimed the Ribis and then pardoned it, according to the Rambam the pardon works. Shibud d'R. Noson does not apply, just like it does not apply to gifts that an Eved Ivri receives when he leaves, for it is called Tzedakah. Likewise, one returns Ribis due to v'Chei Achicha Imach. However, once he claimed to get back the Shochad, Shibud d'R. Noson applies. According to the Ge'onim, just like one cannot pardon Ribis, one cannot pardon Shochad. However, I hold like the SMA that one need not claim Ribis. We say (Bava Kama 94b) that if one returned Ribis, we do not accept from him. Chachamim are unhappy with one who accepts. A Beraisa taught that even if one collected, we return it; they return it to be Yotzei Yedei Shamayim. This connotes that even without a claim, they must return. According to the Taz, why is the lender exempt? Regarding Shochad, surely the judge is exempt before there is a claim.

Nesivos ha'Mishpat (Bi'urim 2): One is not lashed for taking Shochad or Ribis, because the Lav is Nitak l'Aseh (a Mitzvas Aseh corrects it); they can be returned. If so, he must return it even without a claim in order that he not be lashed, like any Lav ha'Nitak l'Aseh. Rather, we must say like the Tumim (Urim 2), that since he gave willingly, when he does not claim, there is a Chazakah that he pardoned. This is unlike the Ketzos ha'Choshen. There is no proof from the Mishneh l'Melech. There, the Chiyuv already began, just he stipulated to pay when he claims from him. Then, Beis Din forces the liable party himself, unlike Shochad or Ribis. The Ge'onim who say that pardon does not help for Ribis surely hold that no claim is needed. There is no proof from Bava Kama 94. Perhaps he claimed already and the lender did not want to return! What he said about Chatas is wrong. If one was forced to buy, it is invalid, for he did not receive money to help him resolve to buy. Every person would like Kaparah, if not for his Yetzer Ra that begrudges the expense of the Korban. Amidst coercion, his Yetzer Ra is weakened (like the Rambam says in Hilchos Gerushin 2:20). Kaparah is like getting money. Tosfos (Bava Basra 48a DH Ileima) says that a Chiyuv Mitzvah is like getting money. A Chiyuv to get Kaparah is like a Chiyuv Mitzvah!

Nesivos ha'Mishpat (Bi'urim 1): If one hired false witnesses, it seems that they need not return what he paid them. Only regarding Ribis and Shochad the Torah said "do not take", and Iy Avid Lo Mehani. The Gemara says so explicitly about Ribis. Abaye says that it must be returned due to v'Chei Achicha Imach. There is no verse for Shochad, so he keeps it. Rava obligates returning it due to Iy Avid Lo Mehani; it is as if he did not take it, and the owner still has it.

Note: This is clear if the money is intact. If it was consumed, it is not obvious to me why Iy Avid Lo Mehani obligates returning it.

Nesivos ha'Mishpat: There is no Lav to hire false witnesses, just a Lav against false testimony. This is like an Esnan given for a forbidden Bi'ah; she need not return it. Regarding a witness hired by Baisusim to testify falsely about the moon, Rashi (Rosh Hashanah 22b DH Ma'atayim) explains that Chachamim told him 'you may keep the money, even though you did not fulfill your stipulation to those who hired you.' This implies that had he fulfilled it, letter of the law he keeps the money.