According to Jewish law, can someone write a will so that his estate will be divided up according to his own wishes and desires after his death?
Torah law (Devarim 21:15-17) makes clear that the double portion belonging to the firstborn cannot be transferred to another child – even to the firstborn child of the preferred wife. According to the Mishna on today’s daf, a father will also be unable to cut one of his sons out of the inheritance. All of these acts are considered to be matneh al mah she-katuv ba-Torah – he is making a condition that negates Torah law – which is considered null and void.
At the same time, the Mishna has a recommendation for a parent who wants to divide up his wealth as he sees fit – he can give his property away to his children or, for that manner, to anyone he wants – while he is still alive. Even if he arranges to give away all of his money so that there is none left in the estate at the time of his death, he has every right to do what he wants with his money when he is alive. The only prohibition would be for someone to try to abrogate the biblical laws of inheritance.
The Gemara points out that the fact that halakha will not let someone choose to will his property to whoever he wants after death appears to negate a principle taught by Rabbi Yehuda, who rules that tenai she-bamamon kayam – that with regard to money matters, a person can choose to make a condition that does not follow Torah laws. The specific example brought by the Gemara is when someone marries a woman and makes it conditional on his not being obligated to clothe or support her – biblically mandated requirements (see Shemot 21:10) – Rabbi Yehuda rules that he can arrange for a marriage under those conditions.
The Gemara distinguishes between the two cases, arguing that in the case of marriage, the woman agrees to the condition. Apparently such a case would not be viewed as an abrogation of Torah law, but as the wife’s choice to relinquish rights that are coming to her.