QUESTION: In the Mishnah, Beis Shamai and Beis Hillel agree that a Shomeres Yavam who inherits property is in full control of that property and she may sell the property as she pleases. They disagree about who gets control of the property if the Shomeres Yavam dies.
Rabah explains that the Mishnah refers to a case in which she inherited the property when she was a Nesu'ah (fully married). When her husband died childless and she became a Shomeres Yavam, her status became subject to a doubt whether she is considered a Nesu'ah or an Arusah. If she has the status of a Nesu'ah, she receives control of only the principle ("Guf," or actual land) of the property. If she has the status of an Arusah, she receives control of the produce ("Peros," or fruits) of the property as well. In practice, she receives control of both the Guf and the Peros, because regardless of the doubt about her status, she definitely owns the Guf of the property (even if she is considered a Nesu'ah), and thus she has a Chazakah on the Guf. That Chazakah dictates that she should get control of the Peros as well because of the principle of "Ein Safek Motzi mi'Yedei Vadai" -- a doubt does not override a certainty; the doubtful ownership of the Peros does not override the certain ownership of the Guf, and thus she gets control of the Peros since she has definite control of the Guf.
However, why -- when she is a Shomeres Yavam whose status is in doubt -- is she given control of the property? Even if the case is decided in her favor and she is deemed to have the status of an Arusah, she still is unable to sell any of the property according to Beis Hillel! The Gemara earlier quotes the Mishnah in Kesuvos (78a) which states that when a woman inherits property as an Arusah, l'Chatchilah she may not sell the Guf according to Beis Hillel (see Chart). Why, then, does the Mishnah here state that she definitely has control over the Guf and may sell it l'Chatchilah?
ANSWER: The BACH (#5) explains that the doubt is not whether the woman is a Nesu'ah or an Arusah. Rather, the doubt is whether she is a Nesu'ah or less than an Arusah. If her relationship to the Yavam is less than that of an Arusah to her husband, she certainly may sell the property because he has no control over it due to the lack of any substantial bond between them. When the Gemara decides the question in her favor, it does not mean that she is considered an Arusah, but rather that she does not have the status even of an Arusah, and thus the Guf and Peros are certainly hers.
The reason for why she may be "less than an Arusah" may be understood as follows. The Gemara in Kesuvos (78a) explains that Beis Hillel rules that an Arusah may not sell her property l'Chatchilah because he is in doubt about whether or not the rights of the husband during Erusin are the same as his rights during Nisu'in. On one hand, since it is likely that the Erusin will culminate with Nisu'in (at which time he will have the full rights to her property), perhaps he should be entitled to the property now during Erusin as well. On the other hand, since there is a possibility that the Erusin will not lead to Nisu'in, and since the bond of Erusin is not as strong as that of Nisu'in, perhaps he does not yet have the same rights to her property as he will have after Nisu'in. Due to this doubt (whether Nisu'in will occur or not), he does not receive the rights to the property. Accordingly, she may not sell the property l'Chatchilah because of the possibility that her husband is entitled to it, but if she does sell the property the sale is valid b'Di'eved. She may not sell it l'Chatchilah because of the doubt about who has control over the property during Erusin, and not because an Arusah has no control over the property.
This raises an obvious question on the Gemara in Kesuvos. If this doubt is the only reason why she may not sell her property l'Chatchilah, why does the Gemara not decide the doubt in her favor based on the Chazakah which she has on the Guf of the property? The doubt should be decided in her favor just as it is decided in her favor in the Sugya here, which says that her entitlement to the Guf is definite while the husband's entitlement is uncertain, and "Ein Safek Motzi mi'Yedei Vadai" -- a doubt does not override a certainty. She should be viewed as a definite Arusah and she should have the right even to sell the property l'Chatchilah. (TOSFOS to Kesuvos 78a, DH Seifa)
TOSFOS there answers that whether the Erusin will culminate with Nisu'in or not is not a real doubt. Rather, it is assumed that the Erusin certainly will culminate with Nisu'in, and thus the husband is granted certain rights in the properties of his Arusah and she is prevented from selling it l'Chatchilah, even though she has a Chazakah on the property. That is, since it is so probable that the Erusin will end with Nisu'in, the rights to the property are given to the husband to prevent her from selling the property.
The Gemara here, however, is not discussing an ordinary Arusah, but a Yevamah. In the case of a Yevamah, it is not certain (or even likely) that the Zikah will end with Yibum and Nisu'in. There is an equal chance that it will end with Chalitzah. Therefore, the husband (Yavam) is not granted rights to the property based on the likelihood that he will marry her, as that likelihood does not exist in the case of a Yevamah. The Yevamah's status is thus in doubt (whether she is considered an Arusah or a Nesu'ah), and the doubt is resolved by the Chazakah she has on the property. Hence, she may sell the property even l'Chatchilah. Even if she has the status of an Arusah, she is not an ordinary Arusah but a Yevamah, and the likelihood that she will marry the man is no greater than the likelihood that she will do Chalitzah. This may be the intention of the Bach when he says that she is not considered even an Arusah; she is not considered an ordinary Arusah who most likely will become a Nesu'ah.