Mishnah in Maseches Kesuvos: (If a person wrote a document to his wife that said, "I have no claim on your property...if she dies, he does not inherit her.) Rabban Shimon ben Gamliel says that he does inherit her, since he made a stipulation against the Torah. (According to Rabban Shimon, it is a Torah law that a husband inherits his wife.)

Rav Yirmiyah citing Rav: That concept (that we ignore the stipulation) is only true for non-monetary laws, but for monetary laws, like here, one can make a stipulation against the Torah.

Question: What is the reasoning of Rav?

Answer: Because only after death he inherits her, but before that, he cannot make a stipulation since the property is not yet in his possession.

R. Yochanan; The Halacha follows Rabban Shimon ben Gamliel.

Proof: R. Yochanan said that if a wife sold her property during her husband's lifetime, the sale should be valid. Why did Chazal say that it is not valid? To prevent her from taking property from her husband and selling it and claiming that it was her's.