Gemara

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

92b (Shmuel): Due to our ignorance, (if a stranger (Levi) 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.

(Ameimar): The Halachah follows Shmuel.

(Rav Ashi): According to this, if her Yavam is a Kohen, she does Chalitzah and is permitted to Levi.

Objection: We should not allow him to profit from his sin!

Correction: Rather, if her Yavam is a Yisrael, Levi gives her a Get, and she is permitted to her Yavam.

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

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

(Mishnah): If David was Mekadesh a woman to take effect (when it will be possible, i.e.) after he or she converts or is freed, or after her husband dies, or after her sister (David's wife) dies, or after she will do Chalitzah, she is not Mekudeshes (even afterwards. If Kidushin Tofsin b'Yevamah, it would take effect.)

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)!

Rishonim

Rif (29a): The Halachah follows Shmuel, who says that due to our ignorance she needs a Get. If her Yavam is a Yisrael and she married Levi (a stranger), Levi gives her a Get, and she is permitted to her Yavam.

Rif: If a Shomeres Yavam accepted Kidushin, if her Yavam is a Yisrael her husband gives to her a Get and she may do Yibum. She may never marry the Mekadesh, lest a sinner profit.

Nimukei Yosef (DH Gemara): The Ritva and his Rebbeyim say that if she became Mekudeshes b'Shogeg, she is permitted to the Yavam (after a Get) or the Mekadesh (after Chalitzah). This is unlike the Ge'onim and the Ra'avad, who explain that our Gemara discusses one who became Mekudeshes totally b'Mezid (without any testimony), and still she is forbidden only to the Mekadesh. She is permitted to the Yavam.

Rambam (Hilchos Ishus 4:14): If a Yevamah became Mekudeshes to a stranger she is Mekudeshes mi'Safek. Chachamim were unsure whether Kidushin Tofsin b'Yevamah, like other Chayavei Lavin, or if Ein Kidushin Tofsin, like Ervah. He divorces with a Get.

Rambam (19): If a Yevamah became Mekudeshes to a stranger, she did not become forbidden to her Yavam. The Mekadesh gives her a Get, and the Yavam does Yibum or Chalitzah. If the Yavam was a Kohen, who cannot marry a divorcee, he does Chalitzah and she leaves the Mekadesh with a Get. He may not marry her, lest he profit through his sin.

Question (Ramban, brought in Magid Mishneh): The Ge'onim say that if she became Mekudeshes, she is forbidden. Why are we more stringent about her than about one who became Mekudeshes based on one witness who said that her husband died?

Answer (Ramban): We are more stringent here because she was Mezid.

Question (Lechem Mishneh): Why is it obvious to the Ramban that regarding an Eshes Ish she is permitted to the Mekadesh? The Gemara does not discuss this! Perhaps he had a source elsewhere.

Rambam (20): If the stranger who was Mekadesh her remarried her (after he divorced her and she did Chalitzah), we do not force him to divorce her.

Rosh (10:3): R. Tam says that when a Yevamah become Mekudeshes l'Shuk, if the Yavam does not want to do Yibum she may stay with her husband. We do not fine for Kidushin even regarding a woman whose husband went overseas (and she heard that he died). Even though people might say that her first husband divorced her and remarries her after she became Mekudeshes, we are stringent with her only when she transgressed (and had Nisu'in). Just like she may return to her husband, if her husband died she may marry the Mekadesh. All the more so we are not more stringent about a Yevamah than about an Eshes Ish. A case occurred in which a Yevamah did not know that there was a Yavam, and she became Mekudeshes. R. Tam permitted her to the Mekadesh. The Gemara said 'we do not allow a sinner to profit' to explain why we do not force a Yavam Kohen to do Chalitzah; he may leave her to be an Agunah forever. If the Kohen willingly does Chalitzah, she may marry the Mekadesh. It is not considered that he profited. If she married l'Shuk she is forbidden to both of them even if the Yavam does Chalitzah willingly. The Gemara asked about a sinner profiting because it thought that we force the Yavam to do Chalitzah. It did not object to why she is permitted to the Mekadesh. BaHaG says that if she became Mekudeshes she gets a Get and is permitted to the Yavam, but not to the Mekadesh, lest a sinner profit. R. Tam's opinion seems primary. If she became Mekudeshes b'Shogeg there is no fine even according to BaHaG. The concern lest one profit through sin does not apply. The practical Halachah is that if she became Mekudeshes b'Mezid she is forbidden to the Mekadesh, lest a sinner profit. If she thought that there is no Yavam, after Chalitzah she is permitted to the Mekadesh.

Yam Shel Shlomo (10:22): Seemingly, even R. Tam could say that the Gemara asked why one who was Mekadesh b'Mezid profits, i.e. is not forbidden to her, like BaHaG. 'Sinner' connotes Mezid! It seems that R. Tam rejects this because the answer, that if the Yavam is a Yisrael the Mekadesh divorces her, is no Chidush. Surely we do not fine the Yavam, who did nothing wrong. It is his option to do Yibum! Rather, the Mekadesh was Shogeg, and even so we force him to divorce (if the Yavam wants to do Yibum) lest he profit.

Poskim

Shulchan Aruch (EH 159:1): If a Yevamah became Mekudeshes to a stranger (Levi) b'Shogeg, he gives her a Get and the Yavam does Yibum or Chalitzah. If the Yavam was a Kohen, who cannot marry a divorcee, she leaves Levi with a Get and the Yavam does Chalitzah. Levi may not marry her, lest he profit through his sin.

Question (Beis Shmuel 1): Most Poskim permit her to Levi if they were Shogeg. The Ramban, Rashba and the Rosh, citing BaHaG, say so explicitly. It seems that also the Rambam holds that the Gemara forbids only when they are Mezid. The Magid Mishneh cites the Ramban without saying that the Rambam disagrees. The Mechaber himself (17:58) permits an Eshes Ish (who heard that her husband died) to the Mekadesh when they were Shogeg. Our case is a decree due to Eshes Ish. Why are the Shulchan Aruch and the Terumas ha'Deshen (cited below) more stringent here?!

Answer (Beis Shmuel): We must say that here 'Shogeg' does not mean that a witness said that her husband died. Rather, there was a mere Kol (rumors).

Shulchan Aruch (ibid.): If Levi married her after he divorced her and she did Chalitzah, we do not force him to divorce her.

Gra (4): She is forbidden to Levi only so he will not profit from his sin. In this case, he did not profit.

Chazon Ish (20:6): According to this, he may marry her l'Chatchilah. Since he divorced her before Nisu'in, this is not a sinner profiting.

Rema: Some say that we force Levi to divorce her because he was Mekadesh her b'Isur. This is only if he was Mezid. If he was Shogeg, i.e. they did not know that there is a Yavam, he must divorce her if the Yavam wants to do Yibum. If he wants to do Chalitzah, she may marry Levi. He must Mekadesh her again after Chalitzah. This opinion is primary.

Beis Shmuel (5): The Rema discusses when the Yavam wants to do Yibum or Chalitzah. Even if he is a Kohen and may not do Yibum, we do not force him to do Chalitzah unless he wants. All Poskim say so, except for the Nimukei Yosef.

Rema (ibid.): Some say that this is only if Levi was totally Shogeg, i.e. they did not know that there is a Yavam. If they knew that there was a Yavam but they heard a Kol that he died, she must leave him.

Source (Terumas ha'Deshen 222, cited in Taz 3): A case occurred in which there was a Kol that the Yavam died. She became Mekudeshes, the Yavam returned and she did Chalitzah, then married the Mekadesh. This is not called Shogeg. Since she knew that there is a Yavam, she may not rely on a mere Kol. The Gemara asked whether or not we may rely on one witness for a Yevamah; Rishonim argue about the Halachah. But no one says that we rely on a mere Kol; it does not change the Chazakah. This is even like R. Tam, who permitted only when she thought that there is no Yavam. And even if R. Tam permits Mezid, the Rosh concludes like R. Chananel and BaHaG, who do not.

Rebuttal (Bach DH u'Mah she'Chosav): We do not separate a woman from her husband due to an argument about a mid'Rabanan matter! R. Tam permits her to Levi even if she became Mekudeshes b'Mezid! Even though Tosfos says that R. Tam permitted an actual case in which she was Shogeg, that just happened to be the case. Also, we infer that even though she was Shogeg, had she done Nisu'in before Chalitzah she would be forbidden. The Rosh says that even BaHaG does not fine if she became Mekudeshes b'Shogeg. This implies that R. Tam permits even if she was Mezid. Also, it is difficult to consider her Mezid because she relied on a Kol. We find that we are stringent to be concerned for a Kol. She thought that we rely on a Kol even to be lenient. The Terumas ha'Deshen cites the Rosh to say that BaHaG makes her leave if she did Nisu'in (after Chalitzah). Our text of the Rosh does not say so, only that she must leave Levi. This implies that Nisu'in is forbidden l'Chatchilah, but if they had Nisu'in after Chalitzah, she may stay. Mahari Veil says so, and also the Mechaber connotes so in the Shulchan Aruch.

Gra (8): She is close to Mezid because normally, a woman checks carefully before remarrying.

Tur: The Rambam (20) says that if Levi was Mekadesh her again after Chalitzah, we do not force him to divorce her. It is implied that this is even if originally he was Mekadesh her b'Mezid.

Beis Shmuel (4): Our text of the Rambam says that she may stay if he married her after Chalitzah. It seems that this is even if he never divorced her.