Explanation #1: [We infer that an expression of inheritance helps, even if the giver is Bari (healthy),] even without a Tenai Beis Din. The Mishnah connotes that if he did not write to her (Kesuvas Benin Dichrin, i.e. that her sons will inherit her Kesuvah), he is obligated, for it is a Tenai Beis Din. However, if he wrote to her, even without Tenai Beis Din he is obligated, for [R. Yochanan ben Brokah] said also that a Bari (can decide who will inherit him);

[Rav Papa asked that] Ein Adam Makneh Davar she'Lo Ba l'Olam (one cannot transfer ownership of something that is not yet in the world). If so, even if he wrote to her (Kesuvas Benin Dichrin), you are forced to say that they acquired only due to Tenai Beis Din;

Also here, even though if he is Bari, [normally, through an expression of inheritance her sons] did not acquire, here they acquired, for it is a Tenai Beis Din!

Observation: Even so, R. Nasan properly told Rebbi "you taught your Mishnah like R. Yochanan ben Brokah", who holds that Yarsun (an expression of inheritance) helps for a Shechiv Mera. Therefore, here, even for a Bari, they acquired due to a Tenai Beis Din;

According to Rabanan, since normally Yarsun does not help even for a Shechiv Mera, they should not have taught here "Yarsun", even due to Tenai Beis Din, rather, "Yasvun (they will get a gift) from Bnei Chorin"! (I.e. if Rabanan did not want the sons to take from Meshubadim, they could have explicitly taught that they get only from Bnei Chorin.)

Explanation #1 (cont.): Abaye answered "because he used the expression Yarsun." I.e. even though they acquired only due to Tenai Beis Din, because Ein Adam Makneh Davar she'Lo Ba l'Olam, in any case we can prove that we say so also for a Bari;

If we did not say so normally for a Bari, the Tana should not have taught here "Yarsun" due to Tenai Beis Din, since elsewhere it does not help;

Rather, he should have taught "Yasvun", and enacted that they take from Meshubadim, or there will be an enactment that they take only from Bnei Chorin.

Conclusion: Rather, surely [R. Yochanan] said also for a Bari.

Explanation #1 (cont.): Abaye retracted and said "what I said was wrong, for the Mishnah teaches [another enactment, that daughters are fed, in an expression of a gift]. If one gives to one (sons) in an expression of inheritance, and to another (daughters) in an expression of a gift, even Chachamim agree [that both take effect];

Really, I can say that R. Yochanan ben Brokah did not say about a regular Bari. If so, R. Nasan properly told Rebbi "you taught your Mishnah like R. Yochanan ben Brokah", for according to him, since Yarsun alone helps for a Shechiv Mera, also for a Bari he taught an expression of a gift and inheritance;

However, Rabanan, who say that elsewhere, Yarsun does not help by itself at all, even for a Shechiv Mera, if so, how did he teach Yarsun?

Even though Yarsun helps because there is an expression of a gift with it, still, he should not have taught Yarsun, since it does not help by itself, only with a gift;

Rather, he should have taught "Yasvun from Bnei Chorin." Really, [I can say that R. Yochanan] did not say about a Bari.

Explanation #2 (Rashbam): It is because he said the expression Yarsun, and this is not Makneh Davar she'Lo Ba l'Olam, since the gift comes only after death, and then the sons are already in the world, and they acquire without a Tenai Beis Din.

Rebuttal: Below (141b), it says that granted, R. Meir said [Adam Makneh Davar she'Lo Ba l'Olam]. Did he say that one can be Makneh to Davar she'Lo Ba l'Olam, even though after she gave birth, the fetus is already in the world! (This shows that it does not help that it will be in the world at the time of the Kinyan.)

Explanation #1: [Since we are unsure whether the same Beis Din enacted both,] it is a Safek whether or not [R. Yochanan] said about a Bari.

Explanation #2 (Ri): He said also about a Bari, since R. Nasan said "you taught your Mishnah like R. Yochanan ben Brokah." (Maharsha - the Gemara did not conclude "Teiku". Also, above Rav Mesharshiya resolved that he said so even for a Bari.)