The Prophet of Allah, sallallaahu ‘alayhi wa sallam, prohibited them from cheating when buying and selling

that the Messenger of Allah, sallallaahu ‘alayhi wa sallam, passed by a heap of grains and put his hand in it, and found some wetness, so he, sallallaahu ‘alayhi wa sallam, said: “What is this, O owner of this merchandise?” The man said: “It has been affected by rain, O Messenger of Allah.” So the Prophet of Allah, sallallaahu ‘alayhi wa sallam, said: “Why did you not put it [the dry portion of it] at the top for the people to see? Whoever cheats is not one of me.”

: “Do not do Tasriyah1 with regards to camels and goats. Whoever purchases such a camel or goat has the choice to milk it, and afterwards, can either keep it, or return it alone with a measurement of dates.”