Can a person make a condition that negates a Torah obligation?
Although in the Mishna (Ketubot 84a) Rabbi Shimon ben Gamliel rejects such a possibility, some suggest that in situations that are purely monetary we may say tenai she-be-mamon kayyam – making a condition on a Torah law that has to do with money is acceptable.
One example that is brought by the Gemara is the case of ona’a (exploitative business practices). According to the Torah (See Vayikra 25:14, 17) business transactions must be fair and one side cannot take advantage of another. Thus, overcharging or underpaying is forbidden by the Torah, and the forbidden profits will need to be returned or the transaction voided. What would be the halakha if someone said to his friend “I am selling this to you on the condition that the rule forbidding ona’a does not apply”? Here we find a disagreement between Shmuel who permits such a condition, permitting the sale and Rav who insists that ona’a still applies.
The source for this discussion of ona’a appears on today’s daf. In truth, it is not only a question of whether a person can make an agreement to dispense with the rules of the Torah with regard to money matters, but also a more basic question of how to define the law of ona’a. Some argue that ona’a has two sides to it. On the one hand there is a question of money, on the other hand there are elements of issur – of forbidden practices – involved, as well. It is therefore possible that even if a person can choose to forgo the Torah rules with regard to money, his agreement to forgo the rule of ona’a will have no legitimacy because of the issur involved.
According to the Rambam (Hilkhot Mekhira 13:3) and the Shulḥan Arukh (Ḥoshen Mishpat 227:21), even though a person cannot make a condition that ona’a does not apply, if he clearly states the true value of the item at the time of sale and explains that the price that he is demanding is out of line with its cost in the marketplace, the buyer can choose to purchase the item without the rules of ona’a applying.