Kapłańska 5:4 Halacha: Mishneh Torah, Oaths & Sefer HaChinukh

When a person takes an oath to fulfill a mitzvah and fails to fulfill it, he is not liable for not fulfilling a sh'vuat bitui. What is implied? A person took an oath to make a lulav or a sukkah, to give charity, or to testify on behalf [of a colleague] if he knew testimony [that could affect him]. If he did not make [these articles], give [the charity], or testify, he is exempt for [not fulfilling his] sh'vuat bitui. For a sh'vuat bitui takes effect only with regard to matters left to one's choice - [i.e., matters that] if he wants to, he may perform and if he does not want to, he need not perform, as implied by [Leviticus 5:4]: "whether he will do harm or do good." Therefore whenever anyone takes an oath to harm another person, he is exempt from a sh'vuat bitui, e.g., he takes an oath to strike so-and-so, to curse him, steal his money, or deliver him to the control of a man of force. [The rationale is that] he is commanded not to do [these things]. It appears to me that he is liable for lashes for taking an oath in vain.

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 will 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.”

To not swear falsely: To not swear falsely, as it is stated (Leviticus 19:12), “And you shall not swear in My name falsely.” And they, may their memory be blessed, explained (Shevout 21a) that this verse warns about an oath of speech. And an oath of speech is what is stated in the Torah (Leviticus 5:4), “Or a soul that swears to express with the lips to do bad or to do good.” And it is divided into four parts — two of the future and two of the past, such as swearing on something that was done or not done, and on something that in the future he will do or will not do. And an oath of speech is only practiced with things that it is possible for a person to do, whether in the past or in the future. How is of the past? “I ate,” or “I did not eat”; and so [too,] “I threw,” or “I did not throw a stone into the sea.” And how is of the future? “I will eat,” or “I will not eat”; or “I will throw,” or “I will not throw.” But with things that have a prevention from the Torah, an oath of speech is not practiced. As an oath only rests upon an optional matter — that if he wants, he does it and if he wants, he does not do it — as it is stated, “to do bad or to do good.” But with any matter of a commandment, there is an obligation upon him to do it. Therefore an oath of speech does not rest upon him, whether in the past or in the future — in the case that he swears to perform a commandment, and he did not perform it; and so [too,] if he swears that he performed a commandment, and he did not perform it. As [just] like a liability [for punishment for a false oath] does not rest upon the matter of a commandment in the future, so too does it not rest upon it in the past. And so is the matter elucidated in its place in Shevuot 27a.