The 249th prohibition is that we are forbidden to swear falsely regarding money that we owe.

The source of this commandment is G‑d's statement,1 "Do not lie to one another."

If, for example, one [falsely] denies possessing an object he was entrusted with, he transgresses the prohibition, "do not deny."2 Should he swear falsely to substantiate his previous denial, he transgress the [present] prohibition "do not lie."

The Sifra says, "What does the Torah mean to teach us by saying 'do not lie'? The verse,3 'and he swears falsely' serves as the punishment. Which verse serves as the actual prohibition?4 The verse, 'do not lie.' "

The details of this mitzvah are explained in the fifth chapter of Sh'vuos, where it is explained that a person who makes a false oath in denying a financial claim transgresses two mitzvos — "do not swear falsely by My name"5 and "do not lie to one another."6