Nedarim, Chapter Ten, Mishnah Seven
Our mishnah discusses whether or not a husband can uphold or annul vows that his wife will take in the future.
1) If a man says to his wife, All vows which you may vow from now until I return from such and such a place behold, they are upheld, he has not said anything.
2) [If he said: All vows which you may vow from now until I return from such and such a place], behold, they are annulled,:
a) Rabbi Eliezer says: they are annulled;
b) The Sages say: they are not annulled.
i) Said Rabbi Eliezer: if he can annul vows which have already had the force of a prohibition, surely he can annul those which have not had the force of prohibition!
ii) They said to him: behold, it is said, Her husband may uphold it, and her husband may annul it (Numbers 30:14), that which has entered the category of upholding, has entered the category of annulment; but that which has not entered the category of upholding, has not entered the category of annulment.
Section one: Everyone agrees that the husband may not legally uphold vows that his wife has not yet taken. If, when he returns from the trip he hears that she has taken a vow and he wants to annul it, he still may do so, despite his earlier statement that her vows were upheld.
Section two: This section discusses the reverse situation, in which the husband wishes to annul vows that his wife takes while he is away. Rabbi Eliezer says this annulment is effective and the Sages say that it is not.
Rabbi Eliezer argues that if a man can annul a vow that has already taken effect, he should be able to annul a vow before it has begun to be effective. A husband can annul vows on the day he hears them even though she vowed earlier (we will learn more about this tomorrow). In other words, even if she vowed earlier and therefore had to keep her vow before it was annulled, on the day that he hears of the vow he may still annul it. From here Rabbi Eliezer concludes that he certainly should be able to annul a vow that has not yet had a prohibitory force.
The Sages respond with a midrash on a verse in Numbers. This verse compares upholding and annulling. Since, as we learned in section one, only a vow which has already been made can be upheld, so too only a vow that has already been made can be annulled. To the Sages their midrash is more authoritative and convincing than Rabbi Eliezers reasoning.