The 248th prohibition is that we are forbidden to falsely deny that we owe money or that we were entrusted with an object.
The source of this commandment is G‑d's statement,1 "Do not deny."
It is explained2 that this refers to a financial claim. The Sifra says, "We learn what the punishment is from the verse,3 'If he denies [illegally withholding another person's money] and swears falsely [...he must pay the principal and add one-fifth.]' Which verse serves as the actual prohibition?4 The verse, 'Do not deny.' "
You are aware that one who [falsely] denies possession of an object he was entrusted with — even if he does not swear falsely — is invalid to act as a witness. This is because he has transgressed the mitzvah, "Do not deny."5
The details of this mitzvah are explained in a number of passages in tractate Sh'vuos.
