Negative commandment · Lo Ta’aseh #250 of 613
Not to cheat in business
שלא יונה במקח וממכר
The verse · Vayikra 25:14
וְכִי־תִמְכְּרוּ מִמְכָּר לַעֲמִיתֶךָ אוֹ קָנֹה מִיַּד עֲמִיתֶךָ אַל־תּוֹנוּ אִישׁ אֶת־אָחִיו
“And if thou sell aught to thy neighbour, or buy aught of thy neighbour’s hand, you shall not defraud one another:”
The Rambam’s words
Not to cheat in business, as [Leviticus 25:14] states: "One man should not cheat his brother."
שלא יונה במקח וממכר, שנאמר "אל תונו, איש את אחיו" (ויקרא כה,יד).
Why · Sefer HaChinuch #337
To not mistreat in buying and selling
“The root of the commandment is well-known, as it is something to which the intellect testifies. And if it were not written, it is appropriate that it would be written — as it is not fitting to take the money of people through falsehood and deceit.”
Who & when
“And [it] is practiced in every place and at all times by males and females.”
Where in the Mishneh Torah
Hilchot Sales הלכות מכירה