Halakhah for Shevuot 41:21

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.

And if you will ask, "And why do we administer lashes for exchange - behold, it is a negative commandment that is rectifiable by a positive commandment, and that [commandment] is, 'and it and its exchange will be consecrated'"; the Sages, may their memory be blessed, have already given a reason for that matter. And they said (Temurah 4b) it is because there are two negative commandments about it, as I have written in this Order (Sefer HaChinukh 352); and a positive commandment does not come and uproot two negative commandments. And they also said another reason - since the negative commandment of exchange is not the same as the positive commandment about it: As if the community, or partners make an exchange, they do not create an exchange (it is not effective), even if they are warned not to exchange (it is forbidden). And since the negative commandment is not the same as the positive commandment, we do not say about it that its law is like a negative commandment that is rectifiable by a positive commandment. And if you will ask further, "And why do we administer lashes for this negative commandment, as it is possible to transgress it without an act, with speech alone - and the principle is established for us that we do not administer lashes for any negative commandment that does not have an act [involved] with it"; we have already written the answer in many places: That they, may their memory be blessed, explicitly excluded (Shevuot 21a) swearing, exchanging and cursing his fellow with [God's] name from this principle. As the Torah was very stringent about them to make them liable for lashes, even though there is no act [involved] with them.