Gemara

(Rav Sheshes, or a Beraisa): If one said 'this Get will not work', or 'it will not permit/abandon/send/or divorce', or 'it should be a shard' or 'like a shard (i.e. worthless)', he words take effect;

If he said 'this Get does not work', 'it does not permit/abandon/send/or divorce', or 'it is a shard/like a shard', his words do not take effect.

Question: If Reuven appointed Shimon to give a Get, and was Mevatel (cancelled) it, can Reuven later use the Get?

Answer #1 (Rav Nachman): He can.

Answer #2 (Rav Sheshes): He cannot.

The Halachah follows Rav Nachman.

Question: R. Yochanan said that if a woman accepted Kidushin to take effect after 30 days, she can retract. The Halachah follows R. Yochanan! (Here also, the Get should become void!)

Answer: There, her initial acceptance was mere words, so her latter words nullify her initial words. Here, the husband nullifies the appointment of the Shali'ach. A Get itself does not become Batel!

Kidushin 59b - Contradiction: The Halachah follows R. Yochanan, and the Halachah follows Rav Nachman!

Answer: The husband was Mevatel the Shelichus. Did he Mevatel the Get itself?!

Gitin 17a (Mishnah - R. Shimon): If a Get was written during the day and signed at night, it is Kosher.

18a (Reish Lakish): R. Shimon is Machshir the Get if it was signed that night, but not if it was signed (after this) during the next 10 days, lest he appeased her.

34a: Rav Sheshes forced a man to (make Sheluchim to) divorce his wife. The man (lied and) told the Sheluchim that Rav Sheshes told them to cancel the Get. Rav Sheshes required them to write another Get.

55a (Rava): If a man told witnesses that he is going to divorce his wife, and he told her to take a loan document (and gave a Get to her), she is divorced.

One might have thought that since he told her that it is a loan document, he was Mevatel it. Rava teaches that this is not so. Had he been Mevatel it, he would have told the witnesses! He deceived her to spare himself shame.

Rishonim

Rif: The Halachah follows Rav Nachman. He can use the Get again.

Rambam (Hilchos Gerushin 6:21): If Reuven sent a Get with a Shali'ach and was Mevatel it, he can use the Get again when he wants. He was Mevatel the Shelichus; he did not Mevatel the Get from being a Get. If Reuven was holding the Get and said that it is Batel, he can never use it. It is like broken earthenware. If he divorced with it, she is not divorced. Similarly, if a Shali'ach had it and he specified that he is Mevatel the Get, he can never use it.

Rebuttal #1 (Ran DH Oh Eino...): If Rav Nachman agreed that it is possible for the Get to be Batel, he would not permit a married woman based on a mere presumption that the husband was Mevatel only the Shelichus. 'It should be a shard' connotes that he was Mevatel the Get itself! We say that he was Mevatel the Shelichus only because the Get cannot become Batel. Rava taught that one who told his wife 'take this loan document' was not Mevatel it, i.e. the giving. The Get cannot be Batel.

R. Akiva Eiger (EH 141:66): The Ran holds that Rav Nachman and Rav Sheshes argue about the Beraisa, which discusses expressions of Bitul when the husband does not have the Get now. Those who argue with the Ran explain that Rav Nachman and Rav Sheshes argue about the Mishnah. They agree with the Ran that the expressions in the Beraisa connote Bitul of the Get more than Bitul of the Shelichus, and Rav Nachman would agree that the Get cannot be used again. All the expressions were taught together, so surely the same law applies to all of them. Alternatively, those who argue with the Ran explain that the Beraisa discusses when the husband has the Get. If so, there is no source that all the expressions connotes Bitul of the Get itself.

Rebuttal #2 (Rashba 32b DH Hachi...): The Gemara compares Bitul of a Get and of Kidushin. Surely money cannot be Batel so that it cannot be Mekadesh!

Rosh (3:4): The words 'Gita Gufei Lo Batil' connote that even if he explicitly said that he is Mevatel the Get, he cannot. Also, we say that giving Kidushin is mere words, but a Get is like an action, and cannot be Batel. However, in Kidushin the Gemara says 'he was Mevatel the Shelichus. Did he Mevatel the Get itself?!' We can reconcile the two texts. 'Did he Mevatel the Get itself?!', i.e. he cannot. Since it was written Lishmah, it is like an action. However, one should not rely on this in practice. Rather, we are stringent.

Tosfos (18b DH Shema): If a Get was not signed the same day or night, we are concerned lest he appeased her. R. Tam explains that he was Mevatel the Get, i.e. the Shelichus of the witnesses to sign it. The Get itself cannot be Batel (if he will later authorize signing it), for the Halachah follows Rav Nachman.

Tosfos (32b DH Rav and DH Hasam): How can Rav Sheshes say that one can be Mevatel the Get itself? All agree (Kidushin 59a) that words cannot be Mevatel a Ma'aseh (action)! We must say that until it reaches her hand, it is not a Ma'aseh. Rav Nachman agrees that he can Mevatel it before she gets it.

Note: What is Tosfos' source that Rav Nachman agrees that it is not a Ma'aseh until she gets it? Perhaps this is the basis of his argument with Rav Sheshes!

Poskim

Shulchan Aruch (EH 141:66): If Reuven sent a Get with a Shali'ach and was Mevatel it, he can use the Get again when he wants. He was Mevatel the Shelichus; he did not Mevatel the Get from being a Get. If Reuven was holding the Get and said that it is Batel, he can never use it. Similarly, if a Shali'ach had it and he specified that he is Mevatel the Get, he may not use it. If he divorced with it, she is Safek divorced.

Beis Yosef (DH ha'Shole'ach): Tosfos, the Ran, Rashba, Rosh and Ramban hold that the Get cannot be Batel, but we should be stringent. The Rambam holds that it can be Batel if he explicitly says so. Therefore, if he divorced with it again, she is Safek divorced.

Gra (131): Some bring a proof for the Rambam from 34a (they needed to write another Get because the first became Batel). The proofs for the Rambam were refuted.

Note: We can say that the man was Mevatel the Get before they signed it, or there was concern lest he say that he was and cast doubt on the Get.