Gemara

(Mishnah): If one told his wife "you are forbidden to me like my mother," we permit it through a different Pesach (than his mother's honor), so he will not take the matter lightly.

Contradiction (Beraisa): If one man told his wife "you are forbidden to me like my mother", or his sister, like Orlah, or Kilayim, this has no effect.

Answer #1 (Abaye): Mid'Oraisa it is void, but mid'Rabanan he must permit it.

Answer #2 (Rava): The Beraisa discusses a Chacham. Our Mishnah discusses an Am Ha'aretz.

Support: Also elsewhere we find that an Am ha'Aretz must permit his vow (lest he take vows lightly), but a Talmid Chacham need not.

21a - Version #1 (Rav Yehudah): One who took one of the four vows must ask a Chacham to permit it.

Version #2 - Rav Yosef (Rav Yehudah citing Rav Asi): A Chacham can permit a vow only if it is like one of the four vows.

He holds that we are not Pose'ach a vow with regret.

A man who had vowed came to Rav Huna to permit it. Rav Huna asked 'do you still desire the vow?'

The man said 'no', and Rav Huna permitted the vow.

A man who had vowed came to Rabah bar Rav Huna to permit the vow.

Rabah bar Rav Huna: If 10 people had appeased you at the time of the vow, would you have vowed?

The man said 'no', and Rabah bar Rav Huna permitted the vow.

(Beraisa - R. Yehudah): If one seeks permission of a vow, we ask if he still desires the vow. If he says 'no', we permit it;

R. Yishmael b'Rebbi Yosi says, in the name of his father, we ask him 'if 10 people had appeased you at the time of the vow, would you have vowed? If he says no, we permit the vow.

A man came to R. Asi to permit a vow. R. Asi asked if he regrets it.

The man (incredulously): No?! (Of course I do!)

R. Asi permitted the vow.

A man came before R. Elazar to permit a vow.

R. Elazar: Are you still happy with the vow?

The man: Had they not angered me, I never would have vowed.

R. Elazar: It should be like you want (the vow is permitted).

22b (Rava): The Halachah is, we are Pose'ach with regret.

Rishonim

The Rif brings our Gemara with minor changes.

Rif (cited in Rosh (3:2) and Ramban (7a). It is not in our version of the Rif): Nowadays we permit vows only similar to (what Rav Yehudah said in the name of) Rav Asi, i.e. if it is like one of the four vows. We permit Shevu'os only to fix matters or for a Mitzvah, e.g. to make Shalom between man and wife or in the Tzibur. We do not permit like Rav Huna and R. Yishmael b'Rebbi Yosi (because he regrets it now, or he would have refrained had people appeased him).

Rosh: The Rif said 'nowadays.' This was a stringency, for in his day people were lax about Nedarim. The Halachah is like I wrote (we permit one who regrets that he ever vowed.)

Rambam (Hilchos Shevu'os 6:5): To permit an oath, one comes to a Chacham and says 'I made this oath, and I regret it. Had I know that I would be in such pain, or that this would occur, I would not have sworn. Had my mind been then like it is now, I would not have sworn. The Chacham asks if he already regrets; the Nishba says 'yes', and the Chacham permits him.

Rambam (10): If one swore and does not regret it and came to Beis Din to affirm his oath, sometimes Beis Din seeks to permit it. This is when permitting it will lead to a Mitzvah or Shalom between man and wife or between men, and fulfillment will cause an Aveirah or quarrels. We are Pose'ach a Pesach and discuss the matter, and inform him of the results of his oath until he regrets it. If he regrets due to their words, we permit him. If not, he fulfills his oath.

Beis Yosef (YD 228 DH umi'Toch): It seems that the Rambam explains like Rashi (Nedarim 21b DH ka'Savar), that 'we are not Pose'ach with regret' means that a Chacham may not suggest 'did you vow with this in mind?' The Noder must regret (by himself) first. (I.e. Pose'ach means to initiate.) We hold that Ein Poschim with regret, but many Mishnayos say that we do suggest regret! Therefore, the Rambam explains that for the sake of a Mitzvah, we are Pose'ach for the Noder.

Rebuttal (ha'Emek She'alah Sof 23, p.148): The SMaG says just like the Rambam's words, but explains like R. Tam, that 'we are Pose'ach with regret' means that regret suffices.

Ran (21b DH Ein): A Pesach is better than regret, for with a Pesach, the Noder says that had he thought about the Pesach, he would not have vowed, so his vow is like a mistake. With regret, he does not say that contemplation at the time would have deterred him. He only says that he vowed out of haste or anger, and now he regrets that he ever vowed.

Ran (22b DH Nodarta): (Rav Schorah came before Rav Nachman with a vow to permit. Rav Nachman strove to find a Pesach, but failed; he did not permit the vow.) Even though Rav Nachman holds that we permit with regret, he knew that Rav Schorah was stringent not to permit through regret. Some say that he had vowed to fast several fasts, and he did not regret from the beginning, lest he forfeit the reward for what he already fasted.

Poskim

Shulchan Aruch (YD 228:1): If one vowed and regrets it, and he cannot find an expert Chacham, he goes to three learned commoners who know how to be Pose'ach.

Taz (2): Even though the Heter is through regret, we require commoners who know how to be Pose'ach, for perhaps they will be able to do so (this is better).

Shulchan Aruch (7): One who regrets the Neder itself does not need a Pesach. It suffices that he wishes that he never vowed.

Beis Yosef (DH u'Mah she'Chosav b'Shem): The Kol Bo is stringent not to permit through regret, for not everyone is proficient in it. Also, sometimes the Noder says that he regrets from the beginning, but really he regrets only from now and onwards. We do permit with a proper Pesach.

Shulchan Aruch (ibid.): Some require a Pesach and regret.

Mordechai (cited in Beis Yosef DH veha'Mordechai): Some say that it suffices to ask the Noder if he regrets. We find that Amora'im permitted without asking whether the regret was from the beginning. I say that they did so after finding a Pesach. The Gemara says that Rav Asi permitted after asking only about regret, even though he requires a Pesach! R. Yosef Bechor Shor requires a Pesach. It is good to be stringent, like him.

Ran (21b DH v'Sharyei): R. Asi permitted after asking only about regret. This is not Rav Asi. (Our text is like the Ran.)

Rema: The custom is to be stringent and to make a Pesach from regret. Since he regrets from the beginning, we say 'had you known that you will regret it, would you have vowed?' He says 'no', and we permit him.