1) WRITING A "GET" PRIOR TO "YIBUM"
QUESTION: Rami bar Chama questions whether a man may write a Get for his Yevamah to give to her later, after he does Yibum and marries her. The Gemara suggests that perhaps such a Get is valid. Since the Yevamah is bound to him, she is considered like an Arusah, and the law in the case of an Arusah is that the man may write a Get which he will give to her (and which will take effect) only after he marries her.
How can the Gemara even suggest such a possibility? There is a fundamental difference between a Yevamah and an Arusah. An Arusah has the status of a married woman mid'Oraisa, and thus a Get may be given to her and render her a Gerushah (divorcee) mid'Oraisa. In contrast, a Yevamah is not considered married mid'Oraisa at all; if her Yavam gives her a Get in that state, the Get is ineffective mid'Oraisa. It was only the Chachamim who instituted that if a Yavam gives his Yevamah a Get, she becomes forbidden to all of the brothers.
The next statement of the Gemara is also puzzling. The Gemara says that the reason why a Get written before Yibum might not take effect is that the Yavam did not yet do Ma'amar with the Yevamah. The Gemara implies that had Ma'amar been done, the Get certainly would be able to take effect after the act of Yibum. Why, though, would the Get take effect? Even if the Yavam did Ma'amar, the Get remains merely a rabbinical enactment and should not be able to take effect after he marries her -- when a Get mid'Oraisa is needed to divorce her. If the Gemara refers to a Get which takes effect only mid'Rabanan (such as to prohibit her from marrying a Kohen), even if the Yavam did not do Ma'amar he still may give the Yevamah a Get mid'Rabanan which takes effect (to prohibit her to the brothers). What difference does it make if the Yavam did Ma'amar?
(a) TOSFOS explains that Rami bar Chama's question indeed is whether a Get written during Zikah works after marriage as a Get mid'Rabanan. That is, the Get certainly does not permit her to remarry. Rather, the Gemara's question is whether the Get disqualifies the woman from marrying a Kohen. Since a Get (mid'Rabanan) which the Yavam gives before the marriage disqualifies her from marrying a Kohen, perhaps a Get which the Yavam writes before the Yibum but gives only after the Yibum also prohibits her from marrying a Kohen.
However, if this is the Gemara's question, the answer should be obvious. Since the Yavam can disqualify her from marrying a Kohen by giving the Get now (before Yibum), it is "in his hands" ("b'Yado") to disqualify her and thus the Get he gives later (after Yibum) should also take effect! What difference does it make if the Get is written before Ma'amar or after Ma'amar?
The TOSFOS YESHANIM and RASHBA explain that the Chachamim instituted the concept of a pre-Ma'amar Get only for the purpose of prohibiting the Yevamah to the brothers. This Get serves an entirely different purpose than a Get given to a woman who is married, or even a Get given to a Yevamah after Ma'amar, both of which serve to sever a bond. The pre-Ma'amar Get serves only to prohibit her to the brothers. The Gemara means that it is not in the Yavam's hands to divorce the Yevamah with a normal Get during the period of Zikah, because the type of Get which the Chachamim instituted for that period is not the type which severs a bond. Accordingly, perhaps it will not be effective after the Yavam and Yevamah are married.
(b) TOSFOS cites the RIVAM who answers that the question of the Gemara is whether or not a Get written before Ma'amar may be used after the Ma'amar (and not after the marriage) in order to revoke the Ma'amar. The Gemara's question is based on the presumption that there is a difference in essence between a Get given before Ma'amar and a Get given after Ma'amar (as explained in the previous answer). Again, the Gemara is discussing whether the Get works mid'Rabanan.
(c) The RASHBA writes that the words of the Gemara support neither of the previous explanations. Rather, the Gemara is asking that perhaps the Get might work on a d'Oraisa level command, and effect a valid divorce, after she is married. This also seems to be the understanding of the RAMBAM (Hilchos Gerushin 3:6) and perhaps RASHI here as well.
The Rashba explains that the Gemara suggests that the fact that Chalitzah may be performed with a Yevamah to break a Zikah indicates that the Yavam has the power of "divorce" even before he does Yibum. It is considered "in his hands" to effect an annulment of the bond of Zikah, since he could severe the bond with Chalitzah. This element of "b'Yado" gives him the authority to write a Get now which will take effect only after Yibum is performed and the marriage bond created.
When the Gemara mentions Ma'amar, it refers to a second question. If the answer to the first question is that it is not considered in his hands to break the bond of Zikah now and thus he may not write a Get to take effect later, perhaps he at least may write a Get before Ma'amar to take effect after Ma'amar which will disqualify her from marrying a Kohen (as Tosfos (in (a) above) explains the question of the Gemara). On the other hand, perhaps such a Get may be written only after Ma'amar. That is the second question of the Gemara.
2) A "GET" FOR "MA'AMAR" AND NOT FOR "ZIKAH"
QUESTION: The Gemara asks whether a Get can be given for Ma'amar and not for Zikah. RASHI (DH O Dilma) explains that if there would exist such a Get, it simply would annul the Ma'amar and restore the Yevamah to the status she had before Ma'amar was done. The Yevamah would be permitted to all of the brothers, while she would be prohibited to the brother who wrote the Get to annul his Ma'amar.
Why does Rashi write that the Yevamah would be prohibited to the brother who wrote the Get? If the Yevamah returns to the status she had before Ma'amar was done, all of the brothers -- including the one who gave the Get -- should be permitted to the Yevamah.
Rashi later (DH Nasan) writes that if the Yevamah is permitted to the brother who gave the Get for Ma'amar, people might err and permit a Yevamah who received a Get for Zikah to marry any of the brothers (in such a case, however, the woman is prohibited to them). Therefore, the Chachamim decreed that a woman who receives a Get for Ma'amar remains forbidden to the brothers, lest people mistakenly assume that a Get given for Zikah permits her to the brothers. (Whether there is such a Gezeirah or not is the subject of dispute earlier (32a) between two versions of a statement of Rava.) This Gezeirah, however, cannot be the basis for Rashi's words. When Rashi here writes that the brother who gave the Get is prohibited to the Yevamah, he cannot mean that the brother is prohibited because of the Gezeirah. If the Chachamim applied such a Gezeirah in this case, then not only should the brother who gave the Get be prohibited to the Yevamah, but all of the other brothers should be prohibited to her as well! If, on the other hand, the Chachamim did not apply such a Gezeirah in this case, then even the brother who did Ma'amar and gave a Get should be permitted to the Yevamah! (TOSFOS DH O Dilma)
ANSWERS: While the Rishonim do not address the source for Rashi's ruling, they do discuss the logical basis for his ruling.
(a) RABEINU AVRAHAM MIN HA'HAR writes that the fact that the brother gave a Get to annul the Ma'amar demonstrates that he has no desire to consummate the Ma'amar and do Yibum with this woman. When Rashi writes that the one who gave the Get cannot do Yibum with the woman, he means that by giving the Get the Yavam shows that he has no desire to "build the house of the dead brother," just as one shows when he gives a Get for the Zikah.
(b) The NIMUKEI YOSEF (32a) explains that the Gemara earlier (12a) teaches that a Ketanah (minor) who did Mi'un with one of the Yevamim after the death of her husband remains prohibited to the brother to whom she did Mi'un, even though her Mi'un uproots the entire Kidushin retroactively. She is permitted to all of the other Yevamim as if she was never married to their brother. Since she did an act to sever the bond with that particular Yavam, the Chachamim decreed that she is prohibited to him because it appears as though she was his "Eshes Achiv," the wife of his brother.
It is evident that when a Yevamah (such as a Ketanah) does an action (Mi'un) to break the bond between her and the Yavam, there is reason to prohibit them to each other. Similarly, in the case of the Gemara here, the Yavam who gave a Get did an act to sever the bond between them; even though his act did not uproot the Zikah, it showed that he did not want to do Yibum with the Yevamah, and thus there is reason to prohibit the Yevamah to him.
(According to both explanations, Rashi here follows the opinion that the Chachamim did not institute a Gezeirah lest people confuse this case with that of a Get given for Zikah (that is, the opinion of the second version of Rava's statement on 32a), even though Rashi in his next comment cites the other opinion that all of the brothers become prohibited because "they might confuse this case with that of a normal Get [given to break the Zikah, and not just the Ma'amar].")