It is commonplace for batei din to allow a man who is in the midst of divorce proceedings to leave the country only after he has a get written, appoints a shaliach (agent to give the get), and gives the get to beit din for safekeeping in case he should not come back. In one such a case, the husband spent a long time abroad and the wife wanted to have the get given. In the interim the husband called and told her: "I will not give the get now. Wait until I return and then I will give it under certain conditions." What is the status of the get in the meantime? While it is possible to nullify the agency, can one not nullify but introduce new conditions?
The Rambam and Tur disagree whether when one made a shaliach and then became a shoteh (mentally incompetent), the shaliach can at that point, fundamentally, continue the process on his own (Rambam) or not (Tur). The Ohr Sameiach explains that the root of the machloket is the question whether when one makes a shaliach, the shaliach just performs the physical actions in place of the affected party or whether the shaliach takes on the status of one who is personally in charge of the matter (Rambam). The K’tzot Hachoshen (188:2) and Rabbi Akiva Eiger (EH 121:3) seem to take that approach in the Rambam’s opinion as well. One can claim that our question of whether the husband can delay or make conditions on the shalaich’s activities depends on that same machloket. However, there are other explanations of the machloket. Rav Shmuel Razovsky says that the question is whether a shoteh is fundamentally unable to divorce or whether there is just a technical problem that can be overcome with the shaliach’s help.
Rav Frank elucidates on an apparent machloket among Rishonim whether the husband’s saying "Do not give the get" is considered an undoing of the agency (Rashba) or at least a temporary suspension of his ability to give the get. The Noda B’Yehuda (EH, II, 127) says that a husband cannot make a new condition in the get after giving it to a shaliach, although he can say that if such and such does not happen he is hereby nullifying the get (see Shem Aryeh 100). It does appear that even if saying "Do not give the get" is not a formal nullification of the get or the agency, if he tells the shaliach to his face not to give the get he may not do so (based on Tosafot, Kiddushin 59b). The Kovetz Ha’arot does explain that a shaliach can only give the get when he does so according to the continued desire of the husband to do so. However, there are strong indications (including based on the Ritva) that once one officially makes a shaliach, he no longer needs to know that the husband wants it given unless there has been at least a semi-formal nullification. However, according to the Rashba, saying "Do not give it," if not a full nullification, is at least a semi-formal suspension of the process and therefore the get should not be given at this time.