QUESTION: The Tana'im in the Mishnah disagree about whether a Yavam may annul the Nedarim of his Yevamah. Rebbi Eliezer maintains that even when there are two Yevamim (two brothers survive the deceased husband), one of them may annul the Yevamah's Nedarim. Rebbi Yehoshua maintains that only when there is one Yavam may he annul the Neder, but not when there are two Yevamim. Rebbi Akiva argues that even a single Yavam may not annul the Yevamah's Nedarim.
The Gemara explains that Rebbi Akiva is of the opinion that there is no "Zikah." Rebbi Yehoshua maintains that although there is "Zikah" and thus a single Yavam may do Hafarah, when there are two Yevamim neither may do Hafarah because "Ein Bereirah" -- it will only become known later which Yavam is her true husband.
The Gemara asks why, according to Rebbi Eliezer, may either Yavam do Hafarah when there are two Yevamim, if the law is "Ein Bereirah"?
What is the Gemara's question on Rebbi Eliezer? Perhaps Rebbi Eliezer rules that "Yesh Bereirah," as indeed some Tana'im and Amora'im rule (see Beitzah 37b, Eruvin 37b)! (MELO HA'RO'IM)
ANSWER: The MELO HA'RO'IM writes that the RAN apparently addresses this question. The Ran explains that with regard to the law of Bereirah, Hafarah differs from Kinyan. In the case of Hafarah, when the husband annuls the Neder it is necessary that it be known that he is her true husband. When the Torah says "v'Ishah Yefeirenu" -- "her husband shall annul it" (Bamidbar 30:14), it teaches that at the time of the Hafarah it must be known beyond a doubt who her husband is. At the moment that one of the two Yevamim annuls the Neder, it is not known who the true husband is; only later does "Bereirah" retroactively determine which Yavam is the true husband. Therefore, neither Yavam may do Hafarah. Even if Rebbi Eliezer rules "Yesh Bereirah," when there are two Yevamim neither one should be able to do Hafarah because at the time of the Hafarah it is not known which one is her real husband (who will do Yibum with her).
The ROSH has a similar approach. The Rosh, however, does not cite the Gezeiras ha'Kasuv of "v'Ishah Yefeirenu," but he suggests that "Zikah" cannot take effect on the Yavam unless it is known at that time that he is the one who will actually do Yibum with the Yevamah. It is not enough to determine it retroactively through "Bereirah." (His logic might be that "Zikah" is not a full bond or connection, but only a tenuous connection between the Yavam and Yevamah. Such a weak connection cannot be applied retroactively, as the Ran says on 67a with regard to the Hafarah of the Arus without the father.)
If this approach is correct, why does the Gemara ask that Rebbi Eliezer should rule that there is no "Zikah" with the Yevamim because of "Ein Bereirah"? The reason why he should rule that there is no "Zikah" is unrelated to "Bereirah," but it is due to a specific requirement in Hafarah!
The Ran avoids this question by pointing out that the words "Ein Bereirah" here do not imply the ordinary meaning of "Bereirah." Rather, these words here mean that it is not clear to which man she is "Zekukah," and therefore there is no "Zikah." A similar use of the phrase "Ein Bereirah" appears in TOSFOS in Gitin (24b, DH l'Eizo) in the name of the RI.