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Rava said: Rabbi Yochanan ben Berokah’s reason is based upon the following verse: It shall be on the day that he bequeaths to his sons. This indicates that the Torah gave authority to a father to bequeath to whom he desires to inherit his estate.
It is brought in the Sifri that the father has such a right even on prospective property, i.e. those that he does not currently own, but they will become his. He can say that he wants property that is not actually in existence yet to be inherited by whomever he desires. Reb Akiva Eiger, however, cites the R”if, who holds that the father cannot give away property that is not yet in existence.
There is another dispute regarding this halachah: The Nesivos quotes a Rit”va, who holds that the father may retract from this stipulation up to the moment he dies. The Ketzos Hachoshen disagrees.
The Mikdash David explains these two arguments with the following chakirah: When the father bequeaths his property to one of the sons, is he establishing that this particular son is his sole inheritor, or are all the children inheritors; the father is merely designating this property that it should belong to this particular son?
If the father is establishing that this particular son is his sole inheritor, this stipulation will be effective even on property that is not yet in existence, for once this son has been established as the inheritor, he will inherit whatever potentially should have belonged to the father. If, however, he is merely designating this property to belong to one of his sons, he can only do that on property which currently belongs to him.The same can be said regarding retraction: If the father is establishing that this particular son is his sole inheritor, he cannot retract from that (he may, however, in the future, add other inheritors, for he is not taking away the inheritor status from this son). If, however, he is merely designating this property to belong to one of his sons, he may retract up until the moment that they actually take possession of the property.