Gemara

(R. Yosi bar Chanina): In Usha they enacted that if a woman sold her Melug property (that she brought into the marriage, and when she is divorced or widowed she gets it back without compensation for change in value) in the life of her husband and she died, he takes the property from the buyers.

Kesuvos 78a (Mishnah): Beis Shamai and Beis Hillel agree that if property fell to a woman after Nisu'in, if she sold it the husband takes it from the buyers.

78b - Suggestion: Our Mishnah teaches Takanas Usha!

Rejection: No. Our Mishnah teaches that he takes the property from the buyers in her lifetime to eat the Peros. The enactment of Usha applies to the land itself, after she dies.

Rishonim

Rif (Kesuvos 18a and Rosh 4:17): In Usha they enacted that if a woman sold her Melug property in the life of her husband and she died, he is Motzi (takes) the property from the buyers. We hold that he need not pay for it. If the money she received for it is around, he must return it. He cannot say 'perhaps it is a Metzi'ah (money that she found).'

Rosh (Kesuvos 4:17): He takes the property for free, for it is as if he bought it first. He does not truly 'take' it, for as long as she was alive, he had the property and ate the Peros. She sold the Guf (permanent rights to it) immediately, and the Peros after she dies. Her husband gets back the Guf, and keeps it permanently. If the money she received for it is around, he must return it. It is theft, since it was not a sale.

Hagahos Ashri: Even if her husband gave to her land, in which case she acquired it and eats the Peros, if she sold it and died, he takes back the property.

Ran (DH ha'Ishah): The Rashba says that he is Motzi the land, i.e. during her lifetime it was not known if she will die first and he will keep it, or if he will die first and the buyer will get it. The Yerushalmi says so. He is Motzi the buyer, i.e. removes him for free. The Rif and Ge'onim concur, for she does not pay at all (Chidushei Anshei Shem - i.e. even if she were alive, she would not pay). We learned that clashes with (married) women are bad. (One who damages them must pay. They cannot pay for what they damage.) The Rashba infers that there are no witnesses that these are the same coins she received. We assume that they are the same. Now that it was revealed that the sale was invalid, the coins were a deposit. If she exchanged the coins, even though what she received for them is in her hands, her husband is exempt. Once she (improperly) used them, they are like a loan. Her husband need not pay her loan, for he inherited from her only Metaltelim.

Ritva (brought in Nimukei Yosef Bava Basra 65a DH Sholach): If what she exchanged the money for is intact, or if she owned property to which her husband had no rights, the buyer can collect in her life or after her death.

Note: The Nimukei Yosef himself (Bava Basra 27a, first wide line) holds that the husband cannot take the land until she dies. It seems that the Ritva holds like the Rosh, who says that he gets back the Guf in her lifetime.

Ran (ibid.): R. Efrayim says that if she sold with his permission, he cannot take from the buyer, for she was like his Shali'ach. A buyer can impose a Cherem on the husband that he is not scheming, i.e. that he did not know about or benefit from her sale.

Rambam (Hilchos Ishus 22:7): A husband eats all Peros of all his wife's property, Nichsei Melug and Nichsei Tzon Barzel, in her lifetime. If she dies in his lifetime, he inherits everything. Therefore, if she sold her Nichsei Melug after Nisu'in, even if the property fell to her before Eirusin, her husband takes the Peros from the buyers all her lifetime. He does not take the land itself, for the Guf of Nichsei Melug is not his until she dies. If she died in his lifetime, he takes the property from the buyers for free. If the money she received for it is around, he must return it. He cannot say 'perhaps it is a Metzi'ah.'

Hagahos Maimoniyos (5): The Gemara says that Takanas Usha enables him to take the land itself after she dies.

Poskim

Shulchan Aruch (EH 90:9): If a woman sold her Nichsei Melug after Nisu'in, even if the property fell to her before Eirusin, her husband takes the Peros from the buyers all her lifetime for free. He does not take the land itself. If she dies in his lifetime, he takes the land itself for free.

Question: In any case the buyer gets no Peros. What does it help to have the land itself?

Answer #1 (Prishah and Drishah 34): It helps for Pruzbul. (We write a Pruzbul only if the lender, borrower or Arev owns land.) It also helps in a case when there is a Safek whether the husband or wife died first (the one holding the land keeps it until the other side brings proof). We cannot say that it helps for the right to spread out produce or live on the land. This is included in eating Peros. The Rema rules like the Nimukei Yosef, who says that the buyer can protest against building or destroying. This is because he holds like the Rambam, who say that the buyer keeps the Guf until she dies. According to the Rosh, the husband holds the land, so the buyer cannot protest, just like she could not protest had she not sold it.

Rebuttal (Chelkas Mechokek 27,30): In any case the buyer can protest. He is in place of her. She could protest even had she not sold it.

Rebuttal (Beis Shmuel 36): In any case the buyer can protest, for perhaps the husband will die first and the buyer will get the land. If we are unsure who died first, all agree that Nichsei Melug go to her heirs (in this case, the buyer).

Answer #2 (Bach Kuntres Acharon DH Kivan): What forced the Prishah to say that having the land helps for these? Surely it is useful for acquiring Metzi'os on it, acquiring through Meshichah (taking something to one's Reshus), acquiring a Get, Bar Meitzra (when one sells land, his neighbors have first right to buy)...

Answer #3 (Prishah 37): If the buyer has the land itself until she dies, he does not get his money back until then.

Rema: Some say that even in her lifetime he takes the land itself for free.

Shulchan Aruch (ibid.): If the very money she received for it is around (Rema - or if money was found in her possession and we can attribute it to this), he returns it to the buyer. He cannot say 'perhaps it is a Metzi'ah.'

Gra (28): Since the sale was invalid, the money is like theft, like it says in Gitin (44b, regarding one who sold a field in Yovel).

Beis Shmuel (38): If she used to buy and sell in the house, he can say that it is his money. Poskim argue about whether or not a husband must pay loans that his wife took before Nisu'in, but all exempt him from what she borrowed after Nisu'in.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): R. Efrayim says that even if her husband admits that he has the money, he need not return it.

Bach (DH u'Mah she'Chosav Rabeinu): Why does R. Efrayim exempt? Perhaps he can tell the buyer 'perhaps you knew that I will take the property, so you intended for a gift, like one who is Mekadesh his sister, according to Shmuel.' The buyer cannot take the money without disproving this. However, all the Poskim disagree.

Rema: If she became widowed or divorced, her sale is valid.

Question (Beis Shmuel 39): Why is the sale valid? One cannot sell Davar she'Lo Ba l'Olam, i.e. what is not yet in his Reshus! The Yerushalmi says that the sale is valid, for it holds that the husband cannot take the land (now).

Chelkas Mechokek (29): Even according to the Rosh, who say that her husband had the land in her lifetime, if she was widowed or divorced, the sale was valid retroactively.

Note: Perhaps this answers the Beis Shmuel's question. The Rosh agrees that retroactively, the Guf was in her Reshus at the time.

Rema: During her entire lifetime, if her husband wants to build or destroy the land, the buyer can protest.

Darchei Moshe (7): The Nimukei Yosef (Bava Kama Sof 31b and Bava Basra Sof 27a) says that the same applies if she sold Nichsei Tzon Barzel. The sale does not take effect until she is widowed or divorced. Since the sale can take effect, if her husband wants to build or destroy the land, the buyer can protest. The Rosh (Teshuvah 40:2) says that if she sold and her husband was quiet, he did not forfeit his rights.