Kesubos 56a - Stipulation against the Torah

The gemara quotes a machlokes r' meir and r' yehuda whether one can make a stipulation against the torah such as "you should be married to me on the condition that i should not owe you clothes, food, and onah". R' meir says it is not binding and r' yehuda says it is binding since it is only regarding monetary issues (Rashi and Ritva argue whether onah qualifies as a monetary matter, since it is similar to the clothes and food which are instituted for her benefit. Tosafos 56b d.h. v'shaminan agrees with rashi that one cannot stipulate to uproot onah).

Tosafos explains that the default position is that if one stipulates against the torah, the act is binding since the stipulation does not have the power to uproot it. The source that a stipulation can uproot the act is learned from being similar to t'nai bnei gad u'bnei reuven, therefore a condition that goes against the Torah since it is not similar to t'nai b'nei gad, it cannot uproot the act so the kiddushin would be binding even though the condition had not been fulfilled.

Tosafos focuses on the opinion of R' Meir, Is it true that one cannot be mochel monetary obligations? Tosafos is clear that actual mechila without using the language of t'nai, one can certainly be mochel anything. Tosafos only questions, how one can be mochel using the language of a t'nai such as "if you do this and this, i will be mochel the damages that you owe me". Tosafos seems to have 3 approaches:

1. R"i - one cannot be mochel using language of a t'nai (with the exception of a loan since the obligation to be was not introduced by the torah, it is not considered uprooting a din torah). However, Tosafos leaves with question from the gemara "if one says rip my coat on the condition that you are exempt, he is patur", how can he uproot the din torah to compensate for damages using the language of t'nai? Perhaps the answer for the R"i is that when the stipulation is made before the damage, since the damagee is allowing it to be done, it does not qualify as a damage at all. The R"i will only say that if the stipulation is made after the damage has been done and the compensation is owed, that the damagee cannot say to the damager "if you do this and this, i will be mochel on the money you owe me for damages".

2. R' Elchanan - One can use a stipulation in the language of "mechila" just not a stipulation in the language of uprooting the din of the torah.

3. Alternatively Tosafos says that the problem of stipulating against the torah by monetary issues only applies when the torah demands that there are certain monetary obligations to a "chalos" and you try to create the chalos without those ingredients i.e. kiddushin includes sh'er k'sus v'ona.