(Mishnah): If a person said, "Ploni (who is unrelated) will inherit me'' and he has a daughter; or he said, "my daughter will inherit me'' and he has a son, it does not take effect, as he stipulated against the Torah.

R. Yochanan ben Beroka: If he said it about someone who could inherit him, his words take effect; about someone who could not inherit him, his words do not take effect.

(Gemara): R. Yochanan ben Beroka was only referring to a son amongst sons or a daughter amongst daughters; but not a daughter amongst sons or a son amongst daughters.

R. Yochanan: The Halachah follows R. Yochanan ben Beroka.

Question (Rebbi to R. Nasan bar Ba): What is the reasoning of R. Yochanan ben Beroka?

R. Ba bar Chiya: Rebbi actually asked him - why does the Halachah follow R. Yochanan ben Beroka?

R. Nasan bar Ba responded: (From your question it seems that you do not agree with R. Yochanan ben Beroka) but surely you agree with him as you taught in a Mishnah in Kesuvos - "Sons that will be born from our marriage will inherit the money of your Kesubah beyond their shares with their (other) brothers etc.'' (The Mishnah teaches that even if this text was not written in her Kesubah, her sons (from a previous marriage) still inherit more than the sons of the current marriage, even though according to the Torah, all sons should inherit equally. This follows R. Yochanan ben Beroka, who said that he may stipulate when it is a son among others sons.)

Rebbi replied: The correct text of the Mishnah should be, "...will take (as a gift) the money'' (rather than 'will inherit the money').

R. Ila: Even according to the original text there is no proof - since Beis Din made the stipulation and there is a principle that whatever Beis Din declare ownerless becomes ownerless, the stipulation is valid.

This must be the case, as even according to the corrected text of "will take'', how could the Kesubah give over property to sons who are not even born yet? Rather, although a person cannot give over something as a gift (to someone is not yet in the world), here he can because of the special authority of Beis Din; so too for an inheritance, even though a person usually cannot bequeath specifically to one son amongst the others, but when done using the authority of Beis Din, it is a valid stipulation.