Gemara

(Rav): If Reuven sold to Shimon 'on condition that you have no claim of Ona'ah (overcharging or undercharging) 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 Tenai is void;

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

Rejection #1: Even R. Yehudah could hold like Rav. R. Yehudah validates a Tenai 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 Tenai 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 Tena'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...'

Kesuvos 83b: Rav holds that a stipulation against the Torah is void!

(Rav): If one says 'I sell this to you on condition that you have no Ona'ah against me', the buyer can claim Ona'ah.

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 Tenai contrary to Torah!

Answer (Shmuel): If he said 'on condition that you have no (claim of) Ona'ah against me', the Tenai 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, 'on condition that you will not Meshamet me' is a valid Tenai, but not 'on condition that Shemitah will not Meshamet' is not.

Rishonim

Rif and Rosh (4:17): If Reuven sold to Shimon 'on condition that you have no claim of Ona'ah against me', Rav says that Shimon 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. If Shimon stipulated and said how much Ona'ah there is, Reuven cannot claim Ona'ah.

Tosfos (51b DH ba'Meh): Even though the Halachah follows Shmuel in monetary laws, R. Chananel rules like Rav, for the Halachah follows Rav in Isurim. This is difficult, for this is not a question of Isur. Surely it is forbidden to deceive. It is a monetary question, whether or not one must return the Ona'ah! The She'altos rules like Rav because the Beraisa supports him. Rav can say that the Beraisa is R. Yehudah. According to Shmuel, the Beraisa is not like R. Yehudah or R. Meir! The Rivan says that Shmuel can explain the Beraisa to mean that Stam is when he did not say 'on condition that', and 'specified' means that he stipulated 'on condition that you have no Ona'ah against me', which is like saying 'I know that it is not worth the price.' Alternatively, one who says 'on condition that you have no Ona'ah against me' normally says 'I know that it is not worth...', even though he is unsure. It is even like R. Meir, for perhaps he does not uproot anything.

Rosh: R. Chananel holds that this is a question of Isur, whether one may stipulate and be exempt from Ona'ah.

Nimukei Yosef (DH Gemara): Levi can stipulate that another will not claim from him (e.g. Onah, or Ona'ah), but he cannot stipulate that there will be no claim. A Tenai 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 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 Tenai is valid.

Question (Lechem Mishneh and Gra CM 227:30): Regarding Shemitah, the Rambam rules like Shmuel, that a Tenai 'on condition that you will not Meshamet me in Shemitah' is valid. The Gemara equated these!.Also all the Poskim (CM 67:3) rule like the Rambam regarding both of these!

Answer (Ketzos ha'Choshen 67:3, citing Keneses ha'Gedolah): We equated Ona'ah and Shemitah according to Shmuel, who holds that a Tenai that Vadai uproots Torah law is Batel. Rav holds that the Tenai helps only if he knows to pardon, i.e. for Shemitah, but not for Ona'ah.

Poskim

Shulchan Aruch (CM 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.

Mabit (2:126): A case occurred in which one stipulated that if the Ona'ah is at least a quarter, there will be a claim of Ona'ah. The Ona'ah was more than a sixth but less than a quarter. Had he stipulated like the Beraisa and Rambam say 'I know that there is Ona'ah...', there would be no Ona'ah. However, the way he stipulated was Asmachta, for he did not know how much Ona'ah there is.

Ketzos ha'Choshen (9 DH v'Chen): When the person does not know to pardon, even pardon of the law is invalid. The buyer can demand the Ona'ah. R. Yehudah holds that the sale is Batel because the Tenai was not fulfilled.

SMA (37): The Ir Shushan says that the Tenai 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 Tenai 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 Tenai. 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.

Machaneh Efrayim (Hilchos Ona'ah 15): If he stipulated 'on condition that you will not claim the Ona'ah', and the other party did not know about the Ona'ah to pardon it, he stipulated contrary to the Torah. If he stipulated 'on condition that you will pardon the Ona'ah', even if the other party did not know about the Ona'ah to pardon it, this is not contrary to Torah. If he does not pardon, the sale is void. The SMA explains that 'on condition that you have no Ona'ah against me' the latter way. This is difficult. Why is the Tenai void, and he must return the Ona'ah? The Tenai is not contrary to Torah. He should say, since you do not pardon, the sale is void! The Beis Yosef says that the Tenai is contrary to Torah, and Kesuvos 83b explicitly says that Rav holds that the Tenai is contrary to Torah! Rather, the Tenai means 'on condition that you will not claim it.' Rashi says that in Bava Metzia, if he says 'on condition that there is no Ona'ah' does not denote pardon. This does not mean that 'on condition that you do not have Ona'ah against me' means that you will pardon it. Rather, you will not claim it.

Shulchan Aruch (ibid.): All the more so, if he said 'on condition that there is no Ona'ah', he has Ona'ah.

SMA (38): The Tur rules like Rashi, that even if the Ona'ah is exactly a sixth, the sale is Batel. The Shulchan Aruch connotes that the law is the same as when he said 'on condition that you have no Ona'ah against me', which is as if he did not stipulate. Ona'ah of a sixth is returned, but the sale stands.

Shulchan Aruch (ibid): 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.

SMA (39): The Shulchan Aruch connotes that it is not enough that he says that he knows that there is Ona'ah. Rather, he must specify how much is the Ona'ah. Even though we hold that one can pardon something without a limit, in any case like this (when the other party knows the Ona'ah), he must specify. If not, the victim can say 'I did not know that there is so much Ona'ah to pardon it.