Gemara

3b (Shmuel): If one lent 'on condition that Shemitah will not be 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 (Rav Anan citing 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, Shemitah is not Meshamet a loan 'on condition that you will not be Meshamet me in Shemitah.' Shemitah is Meshamet a loan 'on condition that Shemitah will not be Meshamet.'

Bava Metzia 51a (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!

Bava Basra 51a (Rav Ashi): The Beraisa discusses a document that says 'my field is sold and given to you.' The owner wanted to give it for a gift. He wrote also 'sold' to accept Acharayus (to compensate the receiver if the land will be taken).

Rishonim

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

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

Rosh (1:3): If one stipulated 'you will not be Meshamet this debt in Shemitah', Shemitah is not Meshamet. If he stipulated 'Shemitah will not be Meshamet', Shemitah is Meshamet.

Rosh (Teshuvah 77:4): If one stipulated that Shemitah will not be Meshamet', it does not help, for he stipulates contrary to Torah. The Torah says that Shemitah is Meshamet! If he stipulated that 'you will not be Meshamet in Shemitah', this applies to the borrower, and it takes effect. If a document says 'he can collect in any way possible, whether Doti (according to Torah law) or not Doti', the latter Tenai is contrary to Torah! Perhaps the intent was 'you will not be Meshamet in Shemitah.' This is contrary to Torah, but we hold like R. Yehudah that such a monetary Tenai is valid. I am unhappy with this, for it does not explicitly say 'you will not be Meshamet.' I do not rule that Shemitah does not nullify it, not do I overturn the custom (where such a Tenai is accepted).

Poskim

Shulchan Aruch (CM 67:9): If one lent and stipulated 'on condition that Shemitah will not be Meshamet', Shemitah is Meshamet. If he lent 'on condition that you will not be Meshamet the loan in Shemitah', his Tenai 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 Tenai is valid. However, he has no control over Shemitah, so a Tenai that Shemitah will not be 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).

Bach (12): He transgressed causing Bitul of Shemitah, and slightly transgressed "lest there be a base thought in your heart" (fear of not being paid after Shemitah), for he would not have lent without the Tenai!

Question: Since a Tenai (that you will not be Meshamet) works, why did Hillel need to enact Pruzbul?

Answer #1 (Bedek ha'Bayis, citing the Ritva): Not everyone remembers to say so at the time of the loan. Also, one does not want to appear as if he despairs of collecting in time. Also, he appears to begrudge the law of Shemitah. Also, the law of Shemitah would be forgotten. Pruzbul reminds people of Shemitah, for all come together to make a Pruzbul.

Answer #2 (Pischei Teshuvah 2, citing Tevu'os Shor): He needed the Tenai for when the borrower would not agree to the Tenai. If David requested a loan from Ploni, and Ploni refused to lend because David did not accept the Tenai, Ploni would transgress refusing to lend due to concern for Shemitah. Pruzbul does not depend on the borrower.

Answer #3 Chasam Sofer (CM 113): In the days of the Gemara, people were careful about Shemitas Kesafim. Nowadays people do not know about it, and the borrower never thought that Shemitah will cancel the loan. It is as if he stipulated 'that you will not be Meshamet.' Modest people write a Pruzbul not to uproot money (take it improperly from the borrower), rather, to avoid Isur. Even one who explicitly made this stipulation transgresses "lest there be a base thought in your heart." If he did not fear, he would not stipulate!

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.'

Rema: Similarly, if the document called the loan 'a deposit', it is not cancelled. This is why he called it a deposit!

Gra (22): This is like Bava Basra 51a.

Urim (19): Why isn't the Tenai (I will not be Meshamet you) Asmachta? He expected to pay before Shemitah (and Shemitah will not apply)! The Rambam holds that Asmachta does not apply to any Tenai of 'on condition that.' The other Meforshim must say that he means that 'if I will not pay you before Shemitah, it is as if it was a deposit' (which is not Mishtament).

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!

Answer #1 (Shach 67:3): Ona'ah is different, for he does not know that there is Ona'ah (so he does not pardon it).

Rebuttal (Gra and Lechem Mishneh): 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 #2 (Drishah 67:12 and Keneses ha'Gedolah Hagahos ha'Tur 67:8, brought in Ketzos ha'Choshen 67:3): The Gemara equates Ona'ah and Shemitah according to Shmuel, who holds that a Tenai that Vadai uproots Torah law is Batel. Rav holds that the Tenai does not help for Ona'ah, for he does not know to pardon. Ona'ah is an Isur, so the Halachah follows Rav. All agree that the Tenai helps for Shemitah.

Ketzos ha'Choshen (227:9 DH v'Chen): When the person does not know to pardon, even pardon of the law is invalid.