הלכה על נדרים 127:1

However, we were not permitted to go out from it [wantonly, but] rather [only] with the stratagem and counsel of the sage; that the one who swore come in front of the man who is wise and understanding of the ways of the Torah and confess to him that [the oath] was from his lack of knowledge - that he did not know at the time that he swore something that he knew afterwards - that he wants to annul what he swore about (Nedarim 71a); and that he recognizes that the smallness of his knowledge and his lacking caused the annulment, not something else or an external thought that would be in his heart, God forbid. And after the confession of his mouth about this, the sage recognizes and sees that there is substance in his words that something new happened to him that if he had had to agree to it at the time that he swore, he would not have sworn and that this is why he regrets [it]; he accepts his confession and he releases him from his oath. And this is what they, may their memory be blessed, said (Berachot 32b), "He cannot forgive [it], but others can forgive it to him." Therefore it is never possible to annul an oath, except with the reason of something new to the one who swore - for example, that he will say, "If I had known thing x, I would never have sworn." As this is like duress. But if he says, "Annul me my oath," without a claim, no man has the power to annul it. And based on this, they, may their memory be blessed, said (Nedarim 64a) that we do not create an opening (to annul the vow) with something new that is not found (that has not happened). As [with this] he does not clearly say that he regrets that he swore - that we should consider it duress - but rather that his will today is like it was at the beginning, but he [just] wants it annulled now. How is this? He swears that he not benefit from x and [x] becomes the town scribe or butcher, and he says, "My will [still] stands that I did not want to benefit from him and I [also] did not want him to become the scribe or the butcher." We do not annul [it] for him until he says, "Since I see that this man has become the scribe, I regret that I swore [off] his benefit forever. And if only I had not sworn!" In this way, we annul [it] for him; as behold, he concedes that his will has changed and that he regrets his deeds completely, due to the lack of his knowledge - as had he known at the time of the oath what he knows today, he never would have sworn. And it is like duress. And we expound (Shevuot 26a), "'A man with an oath' (Leviticus 5:4) - to exclude duress."