The 250th prohibition is that we are forbidden from cheating each other in business when buying and selling.

The source of this prohibition is G‑d's statement1 (exalted be He), "When you sell something to your neighbor, or buy something from your neighbor's hand, do not cheat one another."

The Sifra states, "The verse 'Do not cheat one another' refers to cheating someone monetarily."2

The details of this mitzvah are explained in the 4th chapter of tractate Bava Metzia.3