(Bamidbar 30:9) "And if on the day that her husband hear, he constrain her and annul her vow": We learn vis-à-vis the husband that "constraint" is annulment and vis-à-vis the husband that silence on the day of his hearing is equated with (silence on) the day of the vow.
"and he annul her vow": He can annul the vows that are upon her (i.e., that she has already made), and not the vows that are not upon her (i.e., those that she is destined to make.) R. Eliezer says that he can do so, and that it follows a fortiori, viz.: If where he cannot annul the vows that he has made, he can annul the vows that he is destined to make, then where he can annul the vows that his wife has made, how much more so can he annul the vows that she is destined to make! They (the sages) said to him: No, it is written "and he annul her vow which is upon her." He can annul those vows which she has already made, but not those which are not yet upon her. Variantly (Ibid. 14) "Her husband shall confirm it and her husband shall annul it": What (already) came to confirmation can come to annulment. What did not (yet) come to confirmation (i.e., those vows that she is destined to make) are not subject to (proactive) annulment. "and the L-rd will forgive her": If she vowed and he annulled it in his heart, and she went and broke (what she thought to be a standing vow) wilfully — whence is it derived that she requires forgiveness? From "and the L-rd will forgive her."