Gemara

(Mishnah): If property fell to a Yevamah, Beis Shamai and Beis Hillel agree that she may sell it, and the sale is valid.

Question: If she died, what do we do with her Kesuvah and the property that enters and leaves the marriage with her (Nichsei Melug)?

Answer #1 (Beis Shamai): Her husband's heirs split it with her father's heirs;

Answer #2 (Beis Hillel): The property retains its status quo. The Kesuvah stays by the husband's heirs, and the Nichsei Melug stays by her father's heirs.

Question: Why do Beis Shamai and Beis Hillel argue only in the Seifa?

Answer #1 (Ula): In the Reisha, she fell to Yibum from Kidushin. In the Seifa, she fell from Nisu'in.

Ula holds that Zikah (of one who fell) from Kidushin is like Safek Kidushin, and Zikah from Nisu'in is like Safek Nisu'in.

Objection (Rabah): If so, rather than arguing about the property itself after she dies, they should argue about who eats the Peros during her lifetime!

Answer #2 (Rabah): In both cases she fell from Nisu'in. Zikah from Nisu'in makes Safek Nisu'in.

In the Reisha she is alive. She is Vadai (Kinyan ha'Guf is hers), and her husband is Safek (perhaps he receives Peros). A Safek cannot take from a Vadai. In the Seifa she died and both heirs come to inherit, so they divide.

Question (Abaye): Beis Shamai hold that a Safek can take from a Vadai!

(Mishnah - Beis Shamai): A house fell on Yakov and his son Reuven. Reuven had no money; he owed a creditor. The creditor divides with Yakov's heirs even though they Vadai inherit, and the creditor is Safek.

Answer: Beis Shamai consider a document to be as if it was already collected.

(Mishnah - Beis Shamai): If the husband died before the Sotah drank, she receives her Kesuvah and does not drink.

Question: Why didn't Abaye ask from the Kesuvah in our Mishnah?

Answer: Beis Shamai do not argue (about the Ikar Kesuvah of 200 Zuz);

The Mishnah asked what we do with the Kesuvah, and never answered. Beis Shamai say that heirs of the husband divide with heirs of the father the Nichsei Melug, and Beis Hillel say that the property keeps its Chazakah.

Support (Rav Ashi - Mishnah): Heirs of the husband divide with heirs of the father (i.e. the property was the father's. If it referred to the Kesuvah (which was the husband's), it would have said 'her father's heirs divide with the husband's heirs.'

Answer #3 (to Question (5) - Abaye): In the Reisha, the property fell after she fell to Yibum. In the Seifa, it fell when she was still married.

(Regarding property that falls to her) Abaye holds that her husband's Yad (power to acquire) is like her Yad.

Objection (Rava): If the property fell to her when she was married, all agree that his Yad is stronger than hers!

Answer #4 (Rava): Rather, in both cases, the property fell after she fell to Yibum. In the Reisha, no Ma'amar was given. In the Seifa, a Ma'amar was given.

Rava holds that Beis Shamai hold that Ma'amar makes Vadai Kidushin (to exempt the Tzarah), and Safek Nisu'in (to split her property).

(Rav Papa): Scrutiny shows that the Mishnah is like Abaye. 'Property that enters and leaves with her' connotes property which enters her husband's Reshus, and leaves to return to the father;

However, there is a difficulty. Rather than discussing who gets the property itself after she dies, it should discuss who eats the Peros during her lifetime!

Bava Basra 158b (Mishnah - Beis Shamai): A house fell on Reuven and his wife. His heirs say that she died first; her heirs say that he died first. They divide;

Beis Hillel say, we leave the (Tzon Barzel) property in its Chazakah. His heirs are Muchzak in the Kesuvah, her father's heirs are Muchzak in the Nichsei Melug.

(Gemara) Question: Who is Muchzak in the (Tzon Barzel) property?

Answer #1 (R. Yochanan): His heirs are Muchzak.

Answer #2 (R. Elazar): Her heirs are Muchzak.

Answer #3 (Reish Lakish, and Bar Kapara's Beraisa): Since both have a claim to inherit, they divide it.

Rishonim

The Rif brings our Mishnah verbatim

Ba'al ha'Ma'or (Yevamos 12a): Abaye holds that his Yad is like her Yad. Rashi says that this is according to Beis Hillel. This is wrong. The entire Sugya here seeks to explain Beis Shamai! The Rif does not comment on our Sugya. Perhaps he rules like Abaye, for Rav Papa favors him. However, this is not enough to overturn the Kelal that we follow Rava against Abaye. Further, we asked why Abaye didn't try to prove from our Mishnah that Beis Shamai say that Safek takes from Vadai. He didn't try because Rabah could be Docheh that they do not argue about the Kesuvah. Rav Ashi supports this Dichuy. However, we answered that Beis Shamai consider a document as if it was already collected, and we find (Kesuvos 81a) that Abaye himself accepted this answer! Surely, the conclusion is like Rava. Rabanan rely on Rashi, who says that his hand is better when the husband is alive, and the Yavam's hand equals hers after the husband died. Therefore, if she died the heirs divide the property.

Milchamos Hash-m: It is clear that Beis Hillel hold that Zikah does not make Nisu'in at all. The only question is whether or not the Kesuvah and Tzon Barzel are considered collectable in the husband's lifetime. The Amora'im give various reasons to explain Beis Shamai. Rav Hai Gaon says that we follow Beis Hillel, that the Kesuvah is Muchzak by him, and Nichsei Melug are Muchzakim by her. The Rif agrees.

Rambam (Hilchos Ishus 22:10): If a Shomeres Yavam died, her paternal heirs inherit the Melug property and half the Tzon Barzel. Her husband's heirs inherit the Kesuvah and half the Tzon Barzel.

Rebuttal (Ra'avad): Nichsei Tzon Barzel are totally his. If she dies he inherits her, just like the Ikar Kesuvah. The Rambam was misled by Bava Basra, which says that they split the Tzon Barzel. This does not apply to a Yavam. Whatever is written in the Kesuvah is like the Ikar Kesuvah.

Magid Mishneh: The Rashba says that according to Rashi, if Nichsei Melug fell to her while she was married, they are like Tzon Barzel and the heirs divide them. Some Ge'onim says that in this case they are like Tzon Barzel and his heirs inherit them. The Rambam holds that in any case her heirs inherit the Nichsei Melug.

Rambam (Hilchos Nachalos 3:9): If a Shomeres Yavam died, even if one of the brothers gave a Ma'amar, her father's family inherit the Nichsei Melug and half the Tzon Barzel. Her husband's heirs must bury her, since they inherit her Kesuvah like we explained (in Hilchos Ishus).

Rosh (4:13): In Kesuvos (80b), Rashi says that the Amora'im argue here about the (Tzon Barzel) property like they do in Bava Basra. The Rif rules like Bar Kapara, for he was R. Yochanan's Rebbi. Also, his opinion was the conclusion. R. Tam says that here Beis Hillel and Beis Shamai argue only about Nichsei Melug. We inferred this from 'Property that enters and leaves with her'. There it says simply 'they divide'. This is because if he died first she collected her Kesuvah. Here, she does not collect until Chalitzah, therefore her husband keeps it. Also, if they argue here about Tzon Barzel, the opinion that says that it goes to her hairs gives her heirs greater rights in Tzon Barzel than in Nichsei Melug, which they divide. There are many sources that her heirs have less rights in Tzon Barzel! Rather, all agree that her husband's heirs inherit the Tzon Barzel. It was taught separately from the Kesuvah in order to give the reason, i.e. because they were in his Acharayos. R. Tam's explanation is primary.

Ran (Kesuvos 39a DH Nechasim): Rashi's text says 'The property retains its Chazakah, and the Kesuvah is Muchzak by his heirs. I.e. the property is Nichsei Tzon Barzel, and the Kesuvah refers to 100 or 200 Zuz and Tosefes.

Rosh (ibid.): BaHaG rules like Abaye, who says that his Yad is like her Yad because Rav Papa, who is Basra, supports him. It is not included in YALKGM (the places where the Halachah follows Abaye against Rava), because the argument involves others. Others rule like Rava, because he explains the Reisha and Seifa similarly. It seems that the Rif holds like this. He omitted the opinions here and brought them in Kesuvos (41a); we understand that the Halachah follows Rava.

Poskim

Shulchan Aruch (7): If a Shomeres Yavam died, her heirs inherit her Nichsei Melug and half the Tzon Barzel. Her husband's heirs inherit the Kesuvah and half the Tzon Barzel.

Rema: Some say that his heirs are Muchzakim in all the Tzon Barzel, just like the Kesuvah. We do not take from them if they are Muchzakim.

Rema (6): Some say that Nichsei Melug that fell while she was married are like Tzon Barzel, and they share them. Similarly, they divide her inheritance if she dies.

Beis Yosef (DH v'Chosav ha'Ran, citing Teshuvas ha'Ran 46): We cannot decide which opinion is correct. We cannot take Nichsei Tzon Barzel from the husband against the Ge'onim. If the Nichsei Melug are intact, we give Miktzasam' (some of them) to her heirs.

Note: It is not clear why the Ran says 'Miktzasam'. He rules like Rashi, who says 'Yachloku'. Perhaps the text should say 'Mechtzasam' (half).

Beis Shmuel (10): The Rema omits the opinion of the Ge'onim that he inherits even Nichsei Melug, because her family is Muchzak in them.

Rema (7): In our lands we force to do Chalitzah, so he was never able to do Yibum, so he does not inherit her.

Beis Shmuel (11): Some say that even when we force to do Chalitzah, he inherits her.