Nedarim 15B

ותיובתא דרב יהודה!

And this is a conclusive refutation of the opinion of Rav Yehuda.

כי קתני, דאי איתהני הרי זה ב״בל יחל דברו״.

The Talmud answers: When the Mishnah teaches this, it does not mean that before she went she was permitted to derive benefit from him ab initio. Rather, it means that if she derived benefit from him after the fact, she is in violation of: He should not profane his word.

תנן: ״שאת נהנית לי עד החג אם תלכי לבית אביך עד הפסח״, הלכה לפני הפסח ⁠— אסורה בהנאתו עד החג, ומותרת לילך אחר הפסח.

We learn further on (57b) that if the husband said to his wife: Deriving benefit from me until the festival of Sukkot is konam for you if you go to your father’s house until Passover, then, if she went before Passover, she is prohibited from deriving benefit from him until the festival of Sukkot, as the vow took effect, and she is permitted to go to her father’s house after Passover.

הלכה ⁠— אסורה, לא הלכה ⁠— לא.

The Talmud infers: Only if she went before Passover is it prohibited for her to derive benefit from him. However, if she did not go it is not prohibited for her to do so. She may derive benefit from him, and there is no concern that she will subsequently go to her father’s house, transgressing the prohibition. This is difficult according to the opinion of Rav Yehuda that one may not perform an action that will be rendered retroactively a violation of a vow if the condition is met.

אמר רבא: הוא הדין דאפילו לא הלכה אסורה. הלכה ⁠— אסורה ולוקה, לא הלכה ⁠— אסורה בעלמא.

Rava said: The same is true, that even if she did not go, she is prohibited from deriving benefit from him. The difference is that if she went it is not only prohibited for her to derive benefit from him, but if she does so she is flogged; whereas if she did not go, it is merely prohibited for her to derive benefit from him, lest she violate the vow by going to her father’s house before Passover.