Gemara

83a (Mishnah): If a man wrote to his wife 'I have no claim in your property ... and after your death ... and he does not inherit her;

R. Shimon ben Gamliel says, he inherits her, because he any Tnai contrary to Torah is void.

(Rav): The Halachah follows R. Shimon ben Gamliel, but not for his reason.

R. Shimon holds that a husband's inheritance is mid'Oraisa, and a Tnai contrary to Torah is void. Rav holds that the inheritance is mid'Rabanan, and Chachamim strengthened their enactments as much as Torah law.

Bava Metzi'a 51a (Rav): If Reuven sold to Shimon 'on condition that you have no claim of Ona'ah (overcharging) against me', Shimon can claim the Ona'ah;

(Shmuel): He cannot claim the Ona'ah.

Rav holds like R. Meir, and Shmuel holds like R. Yehudah.

(Beraisa - R. Meir): If Levi told Leah 'you are Mekudeshes to me on condition that you have no claim on me for clothing, food and Onah (Bi'ah at the normal times)', she is Mekudeshes, and his Tnai is void;

R. Yehudah says, the Tnai is valid for monetary matters (clothing and food).

Rejection #1: Even R. Yehudah could hold like Rav. R. Yehudah validates a Tnai when she knows what she forfeits, for she pardons him. Shimon does not know about the Ona'ah in the sale, so he does not pardon it!

Rejection #2: Even R. Meir could hold like Shmuel. R. Meir invalidates a Tnai that definitely uproots Torah law. In the sale, perhaps there is no Ona'ah!

Question (Beraisa): If one says 'on condition that you have no Ona'ah against me', there is no Ona'ah.

If both Tana'im can hold like Rav, like whom is the Beraisa?

Answer #1 (Abaye): Indeed, we must say that Rav holds like R. Meir.

Answer #2 (Rava): The Beraisa is when he specified the amount of Ona'ah (the buyer knows and pardons it). Rav's law is when he said only 'on condition that...'

Makos 3b (Shmuel): If one lent 'on condition that Shemitah will not Meshamet (cancel) the loan', Shemitah is Meshamet;

Question: Regarding a sale 'on condition that you have no Ona'ah', Shmuel invalidates a Tnai contrary to Torah!

Answer (Rav Anan citing Shmuel): If he said 'on condition that you have no (claim of) Ona'ah against me', the Tnai is valid. If he said 'on condition that there is no Ona'ah in the sale', Shmuel agrees that this is contrary to Torah, so it is void.

Likewise, Shemitah is not Meshamet a loan 'on condition that you will not Meshamet me in Shemitah.' Shemitah is Meshamet a loan 'on condition that Shemitah will not Meshamet.'

Rishonim

Rif and Rosh (42a and 9:1): We hold that a monetary Tnai is valid.

Mordechai (294): A monetary Tnai contrary to Torah is valid, but not a Tnai to uproot an enactment mid'Rabanan. Chachamim strengthened their enactments more than Torah law.

Rif (Bava Metzi'a 31a): If Reuven sold to Shimon 'on condition that you have no claim of Ona'ah against me', Rav says that he can claim the Ona'ah. Shmuel says that he cannot. We challenge Rav, and answer that Rav's law is when he said only 'on condition that...', without specifying the amount of Ona'ah.

Nimukei Yosef (DH Gemara): Levi can stipulate that Simchah will not claim from him (e.g. Onah, or Ona'ah), but he cannot stipulate that there will be no claim. A Tnai that the buyer will not return land in Yovel is invalid, for in Yovel the land is not his, it is "Kodesh la'Shem".

Rambam (Hilchos Ishus 6:10): If a man was Mekadesh a woman on condition that he is exempt from giving to her food, clothing or Onah, his Tnai works for food and clothing because these are monetary matters.

Ketzos ha'Choshen (209:11 DH v'Achti): The Gemara (Bava Metzi'a 51a) and the Nimukei Yosef (31a DH b'Mefaresh) connote that the Tnai works only if he asked her to pardon him. However, Rashi (Kesuvos 84a DH Al Menas) holds that a monetary Tnai is valid even if he says that the law should not apply. It seems that the Ramban agrees, for he says (Bava Basra 126b DH Harei) that one cannot become obligated in something against his will. The Gemara in Bava Metzi'a says that R. Yehudah said that the Tnai works only in a case where she knows what she is losing, for she pardons him. Even though we discuss stipulating that the law should not apply, if the person was unaware this is mistaken pardon, which is invalid.

Rambam (Hilchos Shemitah 9:10): If one lent and stipulated that Shemitah will not Meshamet', Shemitah is Meshamet. If he stipulated 'you will not Meshamet this debt in Shemitah', even if he stipulated in Shemitah, the Tnai is valid.

Ri Korkus: The Rambam holds like Rashi, that a Tnai that Shemitah will not Meshamet is invalid because Shemitah is not is his control. A Tnai that 'you will not Meshamet' is valid, because we hold that a monetary Tnai is valid, even if it is contrary to Torah.

Rambam (Hilchos Mechirah 13:3): If Reuven said to Shimon 'on condition that you have no Ona'ah against me', Shimon can claim the Ona'ah. This is Stam, when Shimon does not know how much Ona'ah there is to pardon it. All the more so, if he said 'on condition that there is no Ona'ah', for there is Ona'ah. he has Ona'ah. If he specified there is no Ona'ah, for every monetary Tnai is valid.

Poskim

Rema (EH 38:5): Any Tnai about a non-monetary matter is void. Some say that anything Chachamim decreed about is like something explicit in Torah.

Beis Shmuel (13): The Mordechai learns from Rav, who rules like R. Shimon ben Gamliel, that a Tnai not to inherit one's wife is void. Why does the Rema rule like the Mordechai? All the Poskim disagree. Rather, a Tnai after Nisu'in does not help, just like one cannot stipulate not to inherit his father.

Gra (10): The Halachah does not follow Rav, i.e. we hold that monetary Tana'im are valid even against Torah law. However, the inference that a non-monetary Tnai is void, even regarding mid'Rabanan laws, is true.

Shulchan Aruch (CM 67:9): If one lent and stipulated 'on condition that Shemitah will not Meshamet', Shemitah is Meshamet. If he lent 'on condition that you will not Meshamet the loan in Shemitah', his Tnai is valid. He (the borrower) obligated himself something that the Torah does not obligate him; he is liable.

Beis Yosef (DH V'Afilu): Rashi explains that even though he was Over (transgressed) the Torah, his Tnai is valid. However, he has no control over Shemitah, so a Tnai that Shemitah will not Meshamet is Batel. It seems that the Rambam permits to stipulate, for one may pardon money that the Torah gave to him. Perhaps Rashi agrees, and Over does not mean that he transgressed (rather, he averted Torah law).

SMA (18): In the latter case, it is as if the borrower said 'even though Shemitah is Meshamet and you cannot demand payment, I accept upon myself to pay you.'

Shulchan Aruch (227:21): If Reuven sold to Shimon 'on condition that you have no Ona'ah against me', Shimon can claim the Ona'ah. This is Stam, when Shimon does not know how much Ona'ah there is to pardon it.

Question (Gra 30 and Lechem Mishneh Hilchos Mechirah 13:3): The Rambam, Shulchan Aruch and many Poskim) rule like Rav regarding Ona'ah, that pardon does not help. Regarding Shemitah, they rule like Shmuel, that pardon helps. The Gemara equates these! We cannot say Ona'ah is different, for he does not know to pardon it. Also regarding Shemitah, he does not know whether or not he will pay before Shemitah (Tosfos Makos 3b DH Shmuel)! It is a poor answer to say that here, he does not know if there is Ona'ah at all, but there, he knows the law of Shemitah, just he does not know whether or not it will be paid beforehand.

Answer (Ketzos ha'Choshen 67:3, citing Keneses ha'Gedolah): The Gemara equates Ona'ah and Shemitah according to Shmuel, who holds that a Tnai that Vadai uproots Torah law is Batel. Rav holds that the Tnai helps only if he knows to pardon. Therefore, it helps for Shemitah, but not for Ona'ah.

Ketzos ha'Choshen (9 DH v'Chen): When the person does not know to pardon, even pardon of the law is invalid.

SMA (37): The Ir Shushan says that the Tnai is Batel because it is contrary to Torah. This is wrong. Rather, even if he says that there is Ona'ah, perhaps the buyer thinks that there is only a little.

Defense (Ketzos ha'Choshen 8): The Ir Shushan holds like the Nimukei Yosef, who say that since he does not know to pardon, the Tnai cannot work (without pardon), for it is contrary to Torah.

Nesivos ha'Mishpat (Urim 13): If he stipulated ''on condition that you will pardon the Ona'ah, this is a proper Tanai. I.e., when he finds out about the Ona'ah he will pardon it. If he does not, the sale is Batel. This is not contrary to Torah, for there is no Isur to pardon Ona'ah.

Shulchan Aruch (ibid.): All the more so, if he said 'on condition that there is no Ona'ah', he has Ona'ah. However, if he said 'this is worth 100. I sell it to you for 200 on condition that you have no Ona'ah against me', or is Shimon said 'this is worth 200. I buy it from you for 100 on condition that you have no Ona'ah against me', he has no Ona'ah.