Bava Metzia 51B

אבל הכא מי ידע דמחיל?

But here, where he says: On the condition that you have no claim of exploitation against me, does the other party know that there will be exploitation so that he will consciously waive his rights to claim compensation in the event that there is? He believes that perhaps there will be no exploitation at all.

ושמואל אמר: אנא דאמרי אפילו לרבי מאיר, עד כאן לא קאמר רבי מאיר התם, אלא דודאי קא עקר, אבל הכא מי יימר דקא עקר מידי.

And Shmuel says: I state my opinion even in accordance with the opinion of R' Meir, as R' Meir states his opinion only there, in the case of betrothal, where the husband definitely abrogates Torah law with his condition. But here, who says that either party to the sale will abrogate any Torah law?

אמר רב ענן: לדידי מפרשא לי מיניה דמר שמואל: האומר לחבירו ״על מנת שאין לך עלי אונאה״⁠ – אין לו עליו אונאה. ״על מנת שאין בו אונאה״⁠ – הרי יש בו אונאה.

Rav Anan says: This matter was explained to me personally by Mar Shmuel. In the case of one who says to another: I will be party to this sale on the condition that you have no claim of exploitation against me, the other party does not have a claim of exploitation against him, as one can waive his rights to compensation for the exploitation that he suffered. But if one said: I will be party to this sale on the condition that it is not subject to the halakhot of exploitation, it is subject to the halakhot of exploitation, as in this case it is directly counter to Torah law.

מיתיבי: הנושא והנותן באמנה, והאומר לחבירו ״על מנת שאין לך עלי אונאה״⁠ – אין לו עליו אונאה. לרב דאמר: אנא דאמרי אפילו לרבי יהודה, הא מני?

The Talmud raises an objection from a baraita: In the case of one who conducts business on faith; and in the case of one who says to another: I will be party to this sale on the condition that you have no claim of exploitation against me, the exploited party does not have a claim of exploitation against the one who exploited him. According to Rav, who said: I stated my opinion even in accordance with the opinion of R' Yehuda, in accordance with whose opinion is this baraita, as pursuant to that statement, even R' Yehuda holds that in cases of exploitation one cannot stipulate counter to that which is written in the Torah?