81b----------------------------------------81b

1) ONE WHO SELLS THE PROPERTY OF HIS DECEASED BROTHER
QUESTIONS: The Chachamim instituted that a Yavam may not sell or give away any of the possessions he inherits from his deceased brother, because those possessions are designated for the payment of the Yevamah's Kesuvah.
When the wife is not a Yevamah, although there is a Shibud (lien) on all of a husband's property for the collection of the Kesuvah, he nevertheless is allowed to sell his possessions. In the case of a Yevamah, however, the Chachamim found it necessary to protect the lien of the Kesuvah because the Yavam's own possessions (both present and future) do not become Meshubad towards the payment of the Kesuvah. Since there is less property available for the collection of the Kesuvah, the Chachamim prohibited the Yavam from selling his brother's possessions and causing the Yevamah extra trouble by having to collect it from the buyers.
(If the deceased brother left no possessions, or if nothing remains from them, not even in the hands of a buyer, the Chachamim instituted that the woman may collect her Kesuvah from the Yavam's property. However, if anything remains from the deceased husband's possessions -- even if she must collect it from a buyer -- she may not collect her Kesuvah from the Yavam's property. This explains why the Yavam may not sell the brother's property -- so that the wife should not have to trouble herself to collect it from the hands of the buyers.)
The Gemara discusses what happens if, b'Di'eved, the Yavam sold the property. Rav Yosef says that since the Chachamim enacted that he may not sell it, it is clear that the sale should not take effect. The Gemara questions Rav Yosef's opinion from the Mishnah (78a) that states that according to Beis Hillel, an Arusah may not sell property that she inherits during Erusin, but if, b'Di'eved, she sold it, the sale is valid nonetheless. Although the Arusah violated the enactment of the Chachamim, her sale still takes effect.
(a) Why does the Gemara not explain that Rav Yosef's ruling is related to the principle, "Iy Avid Lo Mehani"? In Temurah (4b), Abaye and Rava argue whether the Kinyan takes effect b'Di'eved or not in a case of a person who tries to make a Kinyan which the Torah prohibits. The same Machlokes should apply when someone transgresses an enactment of the Chachamim and attempts to make a Kinyan which they prohibit. Accordingly, the ruling of Rav Yosef should depend on the Machlokes between Abaye and Rava in Temurah. The opinion which applies "Iy Avid Mehani" (b'Di'eved the Kinyan is valid) in a case of one who transgresses the Torah's command certainly should apply that principle when one transgresses an enactment of the Chachamim, and the Kinyan should be valid, unlike Rav Yosef says.
(b) Why does the Gemara compare the case of a Yavam who sells his deceased brother's property to the case of an Arusah who sells the property she inherits during Erusin? The Gemara earlier (78a) says that an Arusah may not sell the property she receives during Erusin because Erusin is considered Safek Nisu'in (it might have the status of Nisu'in), and thus there is a doubt whether the right to sell the property belongs to her or to him (Rashi 78a, DH Eimar). Since it is a Safek, it is obvious that l'Chatchilah she may not sell it. If she does sell it, b'Di'eved the sale takes effect (because "ha'Motzi me'Chaveiro Alav ha'Re'ayah").
In contrast, in the case of a Yavam who sells his deceased brother's property there is no Safek. Rather, the Chachamim enacted that one may not sell the property of his Yevamah's husband. Therefore, if he does attempt to sell the property, the sale should not take effect. (See PORAS YOSEF.)
ANSWERS:
(a) The HAGAHOS MORDECHAI in Shevuos (end of Perek 2) cited by RAV SHLOMO EIGER (in Gilyon Maharsha) indeed relates Rav Yosef's ruling to the principle of "Iy Avid Lo Mehani" -- a Kinyan is not valid if one transgressed the Torah in order to effect it. According to the Mordechai, it is possible that both Rava and Abaye would agree that one may not make a Kinyan which contradicts an enactment of the Chachamim. (This is because a Takanah d'Rabanan is treated more stringently than Mitzvos of the Torah because "the Chachamim strengthened (made enactments of Chizuk) for their words more than for the [Mitzvos of the] Torah," as the Gemara says earlier (56a).)
However, Rav Shlomo Eiger cites his father, REBBI AKIVA EIGER (Teshuvos 1:129, DH Gam), who cites the PANIM ME'IROS who distinguishes between the case here and the cases in Temurah (in which "Iy Avid Lo Mehani" applies). In the case of the husband's property, the Takanah was made for the benefit of the woman. If the Yavam's sale would be valid, the woman for whom the enactment was made would suffer as a result. Since another person will be affected adversely, it is logical that the Chachamim invalidated the sale even b'Di'eved. In contrast, the Gemara in Terumah discusses cases of Isur v'Heter which do not affect another person. (Although the Gemara in Temurah mentions the case of one who separates Ma'aser before he separates Terumah, which does affect the amount of fruit which a Kohen will receive, in that case no specific person is affected.)
(b) When the Gemara says that an Arusah is a Safek Nesu'ah, it does not mean that her status is in question, as TOSFOS (DH Seifa) mentions. If her status would have been that of a Safek, the Arusah would certainly have been allowed to sell the property even l'Chatchilah since the land (the "Guf") itself belongs to her (even if she is considered a Nesu'ah). The question of her status involves only the rights to sell the produce, but since "Ein Safek Motzi mi'Yedei Vadai" (a doubt does not override a certainty; the doubt that she may be a Nesu'ah does not override the certain fact that she is in possession of the land itself), she may also sell the produce.
Rather, the Gemara means that because there is a possibility that the Erusin will not lead to a full marriage, her husband does not have full rights to what she inherits while she is an Arusah. The Chachamim instituted that she may not sell the property because the husband retains some rights to the property which she inherits. (M. KORNFELD)

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