Petach Ve’charata
Hagaon Horav Dovid Kviat zt”l (Sukkas David, Nedarim 69a, sk 20) finds difficulty with the Radbaz’s answer, pointing out that a sage cannot revoke a vow or a confirmation of a vow without an appropriate petach ve’charata. Sometimes a valid excuse does not present itself until a later time; therefore, it would seem that it is not always possible to obtain a sage’s revocation on the day of confirmation.
A Matter Of Time
Rabbi Kviat (Sukkas David, ibid. sk 21) consequently offers another explanation for the Ran’s position. He notes that even though the husband was not able to annul the vow on Sunday after he had confirmed his wife’s vow, Sunday is still considered “the day he heard” since he could have nullified the vow at the beginning of the day, prior to his approval. In contrast, in the case of the mishnah, Sunday is not considered “the day he heard” since there was no time at all on Sunday, neither at the beginning nor at the end of the day, for the husband to annul the vow, since he didn’t become aware of his ability to nullify the vow until Monday.