Kesuvos 8:8 - 9:1

כל נכסיו – that he inherited from his brothers is surety for her Ketubah.

גירשה אין לה אלא כתובה – but all the while that he did not divorce [her], all of his proerpty is mortgaged to her.

הרי היא ככל הנשים – for a person who divorces his wife and restored her [as his wife In remarriage] on the condition of her first Ketubah he has remarried her and it is necessary to teach us that regarding the widow of a brother who died without issue that she should not say that she is his wife, for he wrote for her a Ketubah from his money, but his widow of a brother who died without issue, who is not his, he wrote for her other than the property of her first husband, which were pledged and surety when he divorced her and restored her [as his wife]. I might sya this is her Ketubah from her. This comes to teach us that this is not the case.

הכותב לאשתו דין ודברים אין לי בנכסיך – while she still is betrothed, he (i.e., the future husband) writes her: “When you marry me, I have [no] claims whatsoever on your property,” even though they were not acquired from him , she an sell them and give them and her sale is valid , for an inheritance that comes to a person of from another place, a man may make a condition upon her that she will not inherit it, and if they acquired I from her, even after she has married, her sale is valid, but he consumes the usufruct, and if she dies, he inherits her, for that is the implication of: “I have no claims on your property,” but their usufruct, I have a claim and as long as they are your property while you are alive, I have no claim on them – but after death, I have a claim on them.

רבי יהודה אומר לעולם הוא אוכל פירי פירות – In the Gemara (Tractate Ketubot 83b), it explains what is usufruct and what is usufruct of the usufruct (i.e., produce vs. yield of the produce)? If she brought in [to the marriage] land and it produced fruit/usufruct, these are usufruct. If he sold these fruits/usufruct and purchased with them fields and they produced fruit/usufruct, these are yield of the produce/usufruct of the usufruct. And when he said to her, I have no claim against your property and their usufruct alone, he can consume the usufruct of the usufruct , according to the words of Rabbi Yehuda, he did not remove himself other than from the usufruct/produce alone and the Halakha is according to Rabbi Yehuda.

רשב"ג אומר אם מתה יירשנה מפני שמתנה על מה שכתוב בתורה – as it is written (Numbers 27:11): “[If his father had no brothers, you shall assign his property to his nearest relative in his own clan,] and he shall inherit it.” From here [we learn] that the husband inherits his wife, but the final result of the matter is that the inheriting of wife by her husband is not from the Torah, but rather from the Rabbis, and the Biblical verse is a mere support, but nevertheless, the Halakha is according to Rabban Shimon ben Gamaliel not because it makes a condition against what is written in the Torah, but rather because the Sages made it stringent measure for the protection of the law (i.e., the Written Torah).