Gemara

(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) and the witnesses 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 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.

Nimukei Yosef (DH Ki): R. Yonah says that if one was coerced until he was Mevatel a Moda'ah, his sale is valid, even if he made a Moda'ah that he is coerced to be Mevatel. The same applies to a Get or gift. Amidst the coercion, he resolved to be Mevatel. The Moda'ah on the Bitul is no stronger than the Moda'ah on the sale. However, one should be stringent about a Get.

Question (Beis Yosef EH 134 DH v'Nimukei): How can he say that the same applies to a Get or gift? If one was coerced to give or divorce, it is invalid!

Rambam (Hilchos Gerushin 6:19): If one told two 'the Get that I write is Batel', and afterwards he wrote it and gave it in front of two others, it is Batel. This is a Moda'ah on a Get. Similarly, if he said 'any Get that I write for my wife Plonis is Batel, and anything that I will say to be Mevatel this Get is Batel', and afterwards he wrote a Get and gave it to her, it is Batel. This is even if he was Mevatel the Moda'ah before he wrote the Get.

Rambam (20): The solution is that the witnesses tell him before the Get is written 'say to us that any words you gave over that would Mevatel this Get, those words are Batel.' He says yes, and commands them to write, sign and give it to her. They do not let him leave until she gets the Get, lest he go and be Mevatel it. One who makes a Moda'ah or is Mevatel it (a Moda'ah) does not need a Kinyan.

Rosh (4:6): Even if he made a Moda'ah and said 'even if I will Mevatel this Moda'ah, the Bitul (and hence the Get) is Batel', if later he was Mevatel Moda'ah and Moda'os on Moda'os to the end of all Moda'os, they are Batel. Just like Bitul helps for one Moda'ah, it helps for many. This is the practice. According to the Rambam, how does his solution help? Perhaps the man made a Moda'ah about (Bitul of the) Moda'ah! Perhaps the Rambam means that Stam Bitul Moda'ah does not help for Moda'ah about a Moda'ah, unless he is explicitly Mevatel all of them.

Rosh (Bava Basra 3:51): In Erchin, we say that a Moda'ah about a Get is valid, even if he was forced and later consented. The Mishnah says "until he says 'I want'", to teach that he must explicitly be Mevatel any Moda'ah. This implies that if he was forced to be Mevatel the Moda'ah, it is a proper Bitul. Even if when he made the Moda'ah, he said 'if I will be Mevatel (this Moda'ah), it should not help', these words do not help. He was forced to be Mevatel everything he said, and amidst coercion he did so.

Poskim

Shulchan Aruch (EH 134:1): One must be Mevatel every Moda'ah before giving a Get.

Shulchan Aruch (2): Therefore, before the Get is written he says 'I am Mevatel any Moda'ah I gave on this Get, and anything whose fulfillment would Mevatel this Get. I testify that I did not say over anything that would disqualify the Get. I disqualify any witness who will testify that I gave over or said anything that will disqualify the Get or weaken its power due to that Moda'ah or words.'

Rema: Some say that it suffices if he is Mevatel Moda'ah and Moda'os on Moda'os to the end of all Moda'os. This is the custom, but it is good to be concerned, and he should say like the first opinion.

Beis Yosef (DH Kosav ha'Ran): The Ran (Kesuvos 41b Sof DH Tanu) says that it does not suffice to write that he is Mevatel all Moda'os 'Ad Olam' (forever). Rather, we must write that he is Mevatel all Moda'os and Moda'os on Moda'os Ad Olam. The Rosh would agree. The Rambam and Rashba do not require this, just that he be Mevatel anything that would disqualify the Get, or disqualify any witness who will testify about a Moda'ah, respectively.

Beis Yosef (DH veha'Rashba): The Rashba says that saying 'I am Mevatel all Moda'os' should suffice. However, to avoid doubts one should be Mevatel any witness who will testify about a Moda'ah. If one gave a Get on condition that he will not come, and said that she is believed about this, we believe her (Gitin 76b). He knows that he cannot disqualify the Get b'Yedei Adam, so he does not seek to ruin her b'Yedei Shamayim, and did not come. Similarly, here he resolves to be Mevatel all Moda'os.

Rebuttal (Ran, cited in Beis Yosef DH veha'Ran...): If he previously gave a Moda'ah that he is coerced to disqualify witnesses on a Moda'ah, the sale is Batel. There is no solution. The Rashba's proof from believing her about a Get is shaky. Chachamim of his generation and the one before us disagreed.

Beis Yosef (DH v'Al): The Magid Mishneh says that the Rambam says that let her husband must stay until she gets the Get, for we follow his last words. Therefore, even if he disqualified witnesses on any Moda'ah, but was Mevatel the Get, it is Batel.

Shulchan Aruch (5): It is a Mitzvah upon Beis Din in every place and at all times to force one who divorces to be Mevatel a Moda'ah that he made.

Beis Yosef (DH v'Lashon...): The Tur connotes that if one was forced properly, even if he made a Moda'ah, the Get is valid. This is wrong. In Erchin, regarding a properly coerced Get, Rav Sheshes requires explicit Bitul of the Moda'ah. If not, it is not a Get. Rather, the Tur means that if he made a Moda'ah, we force him until he is Mevatel it. For this reason, Tashbatz (1:1) concludes that there is a misprint in the Tur, and also because the Tur connotes that if he was Mevatel the Moda'ah, the Get is valid even if he was forced improperly, which is wrong.