Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 245
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 245, with English translation by Maimonides (Rambam).
לדון בדין מקח וממכר, שנאמר: "וכי תמכרו ממכר לעמיתך" (ויקרא כה,יד).
To render judgment [in questions] regarding business transactions, as [Leviticus 22:14] states: "If you sell merchandise to your colleague...."