The Mishneh in Nedorim 90B is about a woman who tells her husband she slept with another man. The Beth Din believes the woman and forces the husband to give her a GET. But times changed, and women were suspected of lying in order to find a new husband. Then the halacha was changed that no woman has the power to force the husband to give her a GET. That is stated clearly in the Mishneh. In Kesubose 63b Tos אבל אמרה says that a woman is not believed to force her husband to give her a GET when she says מאוס עלי because she is suspected of lying in order to find another husband. Thus, whether the woman claims to be forbidden to her husband to force a GET or a woman claims to hate her husband to force a GET, we fear that she wants to find another husband and don't believe her. A prenup that allows a woman to demand a large sum of money to force a GET is also the same, the rabbis do not allow a woman to force a GET as they fear she is lying and just wants another husband.
Someone wants to prove that a prenup is permitted from נחחלת שבעה in my edition page 33 chapter 9. There it is talking about a man who abused his wife and she ran away to her father. The Beth Din will soon gather to judge the man and force him to behave with his wife. But in the interim, say a few weeks, the wife stays with her father and the husband must pay for her food the sum of ten pieces of gold every month until the Beth Din settles the case and the husband behaves. This is not the prenup whereby the husband must sign an obligation to give the wife a huge sum of money or give his wife a GET, because that is designed to force the husband who cannot afford such as monetary loss to give his wife a GET. There the purpose of the money is to force a GET. In the Nachalas Shiva the purpose of the money is to prolong the marriage, so that husband will pay for the wife's food before the Beth Din can deal with the case, and then the Beth Din will force the husband to behave and they will hopefully live happily ever after.
Again, the prenup is forbidden and this is an open Mishneh in Nedorim 90B. Anyone who permits a prenup is somebody who can't read a Mishneh and denies a Tosfose in Kesubose 63b.
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