As we have learned, when a woman takes a neder, it can be nullified – mufar – by her father while she is living in his house, or by her husband once she is married. Similarly, the father or the husband can be meikim the neder, i.e. he can ratify the vow, which will solidify her obligation to fulfill it.
The Mishna on our daf teaches that all agree that there is no significance to a statement made by a man who tells his wife in advance that he ratifies all vows that she takes for the foreseeable future. If, however, he tells her in advance that he is nullifying all of her nedarim, Rabbi Eliezer rules that he has the ability to do so, while the Hakhamim disagree, arguing that he only has the ability to annul her vows after he hears that she has accepted them. Rabbi Eliezer explains his position by pointing out that the ability to annul an existing vow that has already been made would seem to be more powerful than blocking a neder that has not yet come into force. If the husband has the power to nullify a vow we can assume that he certainly has the power to keep it from taking effect.
Why is it so obvious to all that the husband cannot voice his approval before a neder is taken?
Rashi explains simply that a non-existent entity cannot be dealt with. Since the wife has not yet taken any nedarim, there is nothing for the husband to take issue with.
The Ran argues that the reasoning put forward by Rabbi Eliezer in the Mishna indicates why this case will not work. As opposed to his argument regarding hafarat nedarim, the fact that a person can ratify an already existing neder in no way indicates that he will be able to do so for a neder that has not yet been made.