Nedarim 77A

תנן התם: מפירין נדרים בשבת, ונשאלין לנדרים שהן לצורך השבת. איבעיא להו: מפירין נדרים בשבת לצורך השבת, או דלמא אפילו שלא לצורך?

§ We learned in a Mishnah elsewhere (Shabbat 157a): A father or husband may nullify his daughter’s or his wife’s vows on Shabbat and one may request from a halakhic authority to dissolve vows that are for the purpose of Shabbat. A dilemma was raised before the rabbis: May one nullify vows on Shabbat only when they are for the purpose of Shabbat, or may one perhaps nullify vows on Shabbat even when they are not for the purpose of Shabbat?

תא שמע, דתני רב זוטי דבי רב פפי: אין מפירין נדרים אלא לצורך השבת. אמר רב אשי, הא לא תנן הכי: נדרה עם חשיכה ⁠— מפר לה עד שלא תחשך. ואי אמרת לצורך השבת ⁠— אין, שלא לצורך השבת ⁠— לא, מאי איריא חשכה? אפילו בתוך היום אינו יכול להפר דשלא לצורך!

Come and hear the baraita that Rav Zuti from the school of Rav Pappi taught: Vows may be nullified on Shabbat only for the purpose of Shabbat. Rav Ashi said: We did not learn that way in the Mishnah here. The Mishnah teaches: If she took a vow with nightfall approaching, her father or husband can nullify the vow for her only until nightfall. And if you say that with regard to nullification of vows that are for the purpose of Shabbat, yes, he may nullify those vows, but nullifications that are not for the purpose of Shabbat, no, he may not, why does the tanna specify nightfall? After all, even during the day he may not nullify that which is not for the purpose of Shabbat.

תנאי היא: הפרת נדרים כל היום, רבי יוסי ברבי יהודה ורבי אלעזר ברבי שמעון אמרו: מעת לעת.

The Talmud rejects this conclusion: Nullification on Shabbat is subject to a dispute between tanna’im: Nullification of vows can be performed all day on the day that the vow was heard. And R' Yosei, son of R' Yehuda, and R' Elazar, son of R' Shimon, said: A vow can be nullified for a 24-hour period from the time it was heard.