Muwatta Malik (Yahya)

Yahya said: Malik said, regarding a man who buys camels, sheep, cloth, slaves, or anything from goods in bulk: that bulk purchase is not valid in anything that is counted by number.

Yahya said: Malik said regarding a man who gives another man merchandise to sell for him, and its owner has set a price for it, and says: "If you sell it at this price which I have instructed you, then you will have a dinar," or something else that he names for him, upon which they both agree. If he does not sell it, then you have nothing: there is no harm in that, if he names a price at which to sell it, and names a known wage, which he takes if he sells it, and if he does not sell it, then he gets nothing.

Malik said: And similar to that is when a man says to another man: If you are able to capture my runaway slave or bring my stray camel, then you will have such and such. This is from the category of ju‘l (reward), and not from the category of hiring (ijarah), and if it were from the category of hiring, it would not be valid.

Malik said: As for the man who is given merchandise, and it is said to him: "Sell it, and you will have such-and-such for every dinar," for something that is specified, then that is not permissible, because whenever a dinar is reduced from the price of the merchandise, what was specified for him is also reduced from his right. This is uncertainty (gharar); he does not know how much has been assigned to him.

It is narrated from Yahya ibn Sa'id that Muhammad ibn Munkadir used to say: Allah loves the servant who is gentle when selling, gentle when buying, gentle when repaying a debt, and gentle when collecting a debt.