Gemara

(Mishnah - R. Yosi b'Rebbi Yehudah): A case occurred in Tzidon in which Reuven vowed to divorce his wife, and divorced her. Chachamim permitted him to remarry her, for Tikun ha'Olam.

46a: The Mishnah means that one who divorced his wife due to her vow may not remarry her. If he vowed, he may remarry her. R. Yosi b'Rebbi Yehudah cited a case in Tzidon to support this.

46b (Rav Huna): He said 'all produce is forbidden to me if I do not divorce you.'

(Ravina): The Seifa teaches that there was no need to decree for Tikun ha'Olam.

Bava Basra 40b (Chachamim of Neharde'a): If one made a Moda'ah (said that he is being coerced, and does not want what he will do to be valid), if the witnesses on the Moda'ah did not write 'we know the Ones', it is invalid.

This cannot refer to a Moda'ah to invalidate a Get or gift. It is a mere Giluy Milsa (reveals that he is Anus! Since the giver does not receive anything, he would not give the document if he does not want it to take effect, if not for Ones!)

Erchin 21b (Rav Sheshes): A Moda'ah about a Get is valid (the Get is void).

The case is, he was forced and later consented. One might have thought that his consent is a Bitul (retraction) of Moda'ah. This is wrong. If it were a Bitul, the Mishnah could have said only "until he gives a Get". Rather, it says "until he says 'I want'", to teach that he must explicitly be Mevatel any Moda'ah.

Rishonim

The Rif and Rosh (Gitin 16b and 4:6) bring the Gemara in Erchin.

The Rif (Bava Basra 21a) brings the Gemara on 40b.

Rambam (Hilchos Gerushin 6:19): A Moda'ah on a Get is telling two 'the Get that I write is Batel', or 'any Get that Ploni will write for me' or 'that I will write in Ploni's Beis Din' or 'that I will write in the next 20 years is Batel'. If afterwards he wrote it and gave it in front of two others, it is Batel. It does not need a Kinyan.

Magid Mishneh (21): The Rambam holds that Bitul of a Get before it was written is a Moda'ah, and the husband can fix this (by retracting his Moda'ah).

Kesef Mishneh (Hilchos Yibum 4:16 b'Sof): The Rambam holds that Bitul before a Get was written cancels Shelichus of the scribe and witnesses, or it is not Lishmah. Bitul after it was written permanently disqualifies the Get.

Rosh (Bava Basra 3:32): A Moda'ah on a Get or gift is a Giluy Milsa to show that he does not want to do so. Giluy Da'as (revealing his intent) suffices to be Mevatel a gift, all the more so an explicit Moda'ah works. Even if it we know that the Moda'ah is false (there is no Ones), it is Mevatel the gift. If he sold (without Ones), he resolved to sell in order to get the money, and made a Moda'ah in order to get back his property if he will get money later.

Hagahos Ashri (citing the Rashbam): If he did not make a Moda'ah that he is Anus, even if he said that the Get he writes will be Batel, it is not Batel. Since there was no Ones, certainly when he gave it he resolved to divorce. Bitul from before does not help, unless he was Mevatel after it was written.

Poskim

Shulchan Aruch (EH 134:1): If one gave a Moda'ah and told two 'know that the Get that I want to give, I am forced to give it. Therefore, I tell you that it is Batel', it is Batel, even though he did not make a Kinyan.

Beis Yosef (DH v'Af): The Magid Mishneh, Ba'al ha'Itur and Rav Hai Gaon say that the Gemara never mentions Kinyan for a Moda'ah.

Gra (3): We learn from Bava Basra 40a. The Gemara said that Moda'ah, and also Kinyan, must be made in front of two people. Rava had a question about Kinyan. If Moda'ah required a Kinyan, he should have asked also about it!

Beis Shmuel (1): Only an explicit Moda'ah words. Giluy Da'as to give a Moda'ah does not work.

Beis Shmuel (2): According to the Rambam, a Moda'ah works even if he admitted that there is no Ones.

Rema: This is even if he did not tell the two together, rather, one after the other, as long as they are valid witnesses.

Shulchan Aruch (ibid.): The Moda'ah works even if they do not know his Ones, and even if the Ones he claims is false.

Beis Yosef (DH Af...): The Rosh, Ran, Magid Mishneh, R. Yerucham and R. Yonah say that even a false Moda'ah works. The Rashbam says that if there was no Ones, it is not a Bitul. We know that when he gave it, he resolved to divorce. Bitul helps only after it was written. The Ramban agrees. If he was Mevatel the scribe and witnesses, and asked them later, he retracted and made them Sheluchim. Words nullify words, especially to write a Get, which does not need real Shelichus. Once a Get is written, he cannot be Mevatel it. However, one should be stringent. Perhaps he cancelled Shelichus of the scribe and witnesses, and it is as if they wrote without his command, i.e. Lo Lishmah. When he later said 'write it', he meant 'if you want.' He did not say that he retracts from his Bitul. Perhaps it depends on his words. If he said 'it is Batel', it is Batel. If he said 'I am Anus', he was not Mevatel it. Even if it is a Giluy Da'as (that he does not want the Get), we hold that Giluy Da'as does not help. Therefore, the Get is Batel only because we believe that he is Anus.

Beis Yosef (DH veha'Ran): The Ran rules unlike the Ramban. Whenever we know the Ones, the Get or gift is Batel, even if he did not make a Moda'ah.

Shulchan Aruch (ibid.): Even if he gave the Get later, we do not say that he was Mevatel the Moda'ah that he gave, unless he explicitly was Mevatel the Moda'ah. Then, it is Batel. No Kinyan is needed.

Shulchan Aruch (5): A forced Get is Batel only if he made a Moda'ah. If he did not, if he was forced improperly the Get is Pasul. If Beis Din forced him properly, e.g. he was obligated to divorce but refused to, it is valid.

Beis Yosef (DH Kosav ha'Rav): R. Maimon No'ar says that if one agreed to pay a fine to the mayor if he will not divorce, and later he divorced and was Mevatel all Moda'os, it is valid. It is not a coerced Get, for he willingly fined himself. If one forbade Peros to himself if he will not divorce, the Get is not coerced! However, if one swore to divorce this is like Ones, for the Get depends on his oath. If the Get depends on something else, it is not Ones.

Beis Yosef DH bi'Teshuvas): A case occurred in which Reuven and his wife's family agreed to a large fine if he will not divorce by a certain day. He regretted. They warned him about the fine. He tried to compromise with the Gizbar, but could not. Due to fear, he divorced, without Bitul Moda'os. The Rashba (4:40) ruled that if we know about the Ones (her family schemed to get the Gizbar not to agree, and threatened to imprison him until he pays), the Get is coerced and is Pasul. One who receives money resolves to sell. If he was forced to give, no Moda'ah is needed if we know about the Ones. Also here, he gets nothing. Exemption from a fine is not like receiving money. Anyone who divorces is exempted from giving food and clothing, yet it is not considered as if he received.

Shulchan Aruch (7): If Yisre'elim improperly forced one to divorce and he said 'I want', and he was Mevatel the Moda'ah or there was no Moda'ah, the Get is Pasul, but she is forbidden to Kehunah.

Beis Yosef (DH veshe'Lo): If he made a Moda'ah and was not Mevatel it, she is permitted to Kehunah.

Shulchan Aruch (8): If Nochrim forced one to divorce, if he was obligated to divorce, the Get is Pasul, but she is forbidden to Kehunah. If he was not obligated to divorce, she is permitted to Kehunah.

Rema: Even if he was paid to give the Get, we do not say that due to this he consented.

Beis Shmuel (14): The Rashbam says that a Moda'ah on a Get is a Giluy Milsa because he receives nothing. This implies that if he is paid, the witnesses must know the Ones. The Rashbam could agree to the Rema's law. The Rashbam taught only that when we do not know about an Ones, a Moda'ah helps if he was not paid. We hold that a Moda'ah works even if the Ones was false. According to the Rashbam, this is only if he was not paid.