Gemara

(Rav): "Lo SiHYeH Eshes ha'Mes Chutzah l'Ish Zar" - HavaYaH (Kidushin) does not take effect on a Yevamah l'Shuk.

(Shmuel): Due to our ignorance, (if someone else was Mekadesh her) she needs a Get (to marry anyone else).

Shmuel is unsure whether "Lo Sihyeh" teaches that this is a Lav, or that Ein Kidushin Tofsin (Kidushin does not take effect).

(Ameimar): The Halachah follows Shmuel.

(R. Yanai): They voted and concluded that Ein Kidushin Tofsin with a Yevamah.

Question (R. Yochanan): A Mishnah teaches this!

(Mishnah): In the following cases, Leah is not Mekudeshes. David was Mekadesh her to take effect (when it will be possible, i.e.) after he or she converts or is freed, or after the death or her husband or her sister (David's wife), or after she will do Chalitzah. (If Kidushin were Tofsin b'Yevamah, she would be Mekudeshes in the last case.)

Answer (R. Yanai): Correct! Had I not taught to you the law, you would not have seen that the Mishnah teaches it!

(Reish Lakish to R. Yochanan): A great man praised you. If not, I would have said that the Mishnah is like R. Akiva, who says that Ein Kidushin Tofsin with Chayavei Lavin (but Chachamim say that Kidushin Tofsin b'Yevamah)!

Question: The Mishnah cannot be R. Akiva. He holds that a person may acquire Davar she'Lo Ba l'Olam!

(Mishnah): If a woman said to her husband 'what I produce is Konam (forbidden like a Korban) to your mouth', he need not annul this vow;

R. Akiva says, he must annul it, lest she produce more than she is obligated. (Even though her productivity is not yet in the world, her vow takes effect!)

Answer (Rav Huna brei d'Rav Yehoshua): The case is, she said 'My hands are sanctified to their Maker'. Her hands are in the world.

This argues with Rav Nachman bar Yitzchak, who says that R. Akiva holds that a person can acquire Davar she'Lo Ba l'Olam.

Rishonim

Rambam (Hilchos Ishus 7:15): If one said to a Yevamah 'you are Mekudeshes to me after your Yavam will do Chalitzah to you', she is Mekudeshes, because Kidushin Tofsin in her due to Safek.

Tosfos Ri ha'Zaken (Kidushin Sof 62b): Even though the Rambam says that she is Mekudeshes, he means only that we are stringent to require a Get. Surely we are not lenient!

Magid Mishneh: Since we hold that Kidushin Tofsin b'Yevamah mi'Safek, also in this case she is Mekudeshes mi'Safek.

Kesef Mishneh: Perhaps this was the Magid Mishneh's text in the Rambam. Our texts say that she is Mekudeshes, i.e. totally. Since now Kidushin Tofsin in her mi'Safek, this is not Davar she'Lo Ba l'Olam, so Kidushin takes effect totally after Chalitzah.

Rebuttal (Mishneh l'Melech): How can she be totally Mekudeshes, and if someone else was Mekadesh her afterwards we ignore it? Perhaps the Halachah is that Ein Kidushin Tofsin b'Yevamah. If so, she is not Mekudeshes at all!

Defense #1 (Oneg Yom Tov 142 DH v'Im Kenim): The Rambam holds that Machazikim me'Isur l'Isur. I.e. if something was forbidden and we are unsure whether or not a new Isur came immediately when the first Isur left, we consider it Vadai forbidden. The Yevamah was Vadai forbidden to Zarim before Chalitzah, and we are unsure whether or not the Kidushin took effect immediately after Chalitzah, so we consider her Vadai Asur (Mekudeshes).

Defense #2 (Yad ha'Melech DH v'Chen): A person cannot acquire Davar she'Lo Ba l'Olam because the giver and receiver lack Gemiras Da'as, i.e. they do not decide absolutely because the matter is not yet relevant. However, when there is a very strong desire or Hana'ah, the normal requirements do not apply. If a thief sold stolen property, he can Makneh it to the buyer to take effect after he will buy it. An Arev obligates himself without a Kinyan because of the Hana'ah that he causes the lender to lend. If Davar she'Lo Ba l'Olam takes effect even somewhat (e.g. mi'Safek), the parties have Gemiras Da'as and it takes effect totally when it is Ba' l'Olam.

Defense #3 (Noda bi'Yehudah 2 EH 54:11): The Kesef Mishneh's explanation is astounding. We cannot learn from it to other cases. Perhaps we can understand it as follows. A minor can make Kidushin to take effect when he grows up because he is able to acquire now, when an adult is Makneh to him. The Torah taught that the Kidushin cannot take effect until he matures. We do not extend this to teach that the act of Kidushin must be after maturity. According to Chachamim, Kidushin Tofsin b'Chayavei Lavin. Rav holds that the Torah taught that Kidushin cannot take effect on a Yevamah before Chalitzah. Perhaps we do not learn that the act of Kidushin must be after Chalitzah. R. Yochanan brought a proof from the Mishnah, for he holds that R. Akiva (who requires a husband to be Mefer his wife's Konam on Ma'ase Yadayim, lest she produce extra) holds that person can acquire Davar she'Lo Ba l'Olam, so the Mishnah must be Chachamim. Shmuel and Reish Lakish establish the Mishnah like R. Akiva. They hold like Rav Huna brei d'Rav Yehoshua, that in the Mishnah of Konam she was Mekadesh her hands, and R. Akiva holds that a person cannot acquire Davar she'Lo Ba l'Olam. There are two Sefekos to say that Kidushin to take effect after Chalitzah is Tofes totally on a Yevamah. Perhaps Kidushin Tofsin now. And even if not, perhaps the Halachah follows Rav Huna, and Chachamim say that only the Chalos cannot be before Chalitzah.

Rosh (Kidushin 3:7): If David was Mekadesh a woman to take effect after he or she converts or is freed, or after her husband dies, or after her sister dies, or after she will do Chalitzah, she is not Mekudeshes. This Tana holds that Ein Kidushin Tofsin b'Yevamah. The Halachah does not follow him.

Note: R. Yerucham (Nesiv 22:4 189a) says that also the Rif says so. I did not find this in the Rif.

Ran (Kidushin 26a first wide line): A Mishnah teaches that if one was Mekadesh a woman to take effect... or after she will do Chalitzah, she is not Mekudeshes. This is because it is Davar she'Lo Ba l'Olam; now he cannot Mekadesh her. This is not difficult for Rav. Shmuel must say that this is like R. Akiva. We hold like Shmuel, and unlike R. Akiva. Therefore, she is Safek Mekudeshes, for perhaps Kidushin Tofsin b'Yevamah, and this is like Kidushin to take effect after 30 days.

Poskim

Shulchan Aruch (EH 40:6): If one said to a Yevamah 'you are Mekudeshes to me after your Yavam will do Chalitzah to you', she is Safek Mekudeshes.

Chelkas Mechokek (11): The same applies if he was Mekadesh her immediately. The Chidush is that even if he says 'after you do Chalitzah' it is only Safek Kidushin.