Al-Muwatta Hadith 32.7

Yahya said that Malik said, "No one should make a qirad loan except in coin, because the loan must not be in wares, since loaning wares can only be worked in one of two ways: Either the owner of the wares says to the borrower, 'Take these wares and sell them. Buy and sell with the capital realized according to qirad.' The investor stipulates increase for himself from the sale of his goods and what relieves him of expenses in selling it. Or else he says, 'Barter with these goods and sell. When you are through, buy for me the like of my goods which I gave you. If there is increase, it is between you and me. 'It may happen that the investor gives the goods to the agent at a time in which they are in demand and expensive, and then the agent returns them while they are cheap and he might have bought them for only a third of the original price or even less than that. The agent then has a profit of half the amount by which the price of the wares has decreased as his portion of the profit. Or he might take the wares at a time when their price is low, and make use of them until he has a lot of money. Then those wares become expensive and their price rises when he returns them, so he buys them for all that he has so that all his work and concern have been in vain. This is an uncertain transaction and is not good. If, however, that is not known until it has happened, then the wage an agent in qirad would be paid for selling that, is looked at and he is given it for his concern. Then the money is qirad from the day the money became cash and collected as coin and it is returned as a qirad like that."