Buying and Selling: Usury (Ribā) in Transactions Involving Commodities of the Same Kind
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2080
If a commodity that is sold by weight or measure is sold for a greater weight or measure of the same commodity – for example, 1 kilogram of wheat is sold for 1.5 kilograms of wheat – it is usury (ribā) and unlawful. In fact, if one of two commodities is without defect and the other is defective, or the quality of one of them is good and the other is bad, or they are different to one another in price – then, in the event that the seller receives more than he gives, it is still usury and unlawful. Therefore, if a person gives unbroken copper and receives a greater amount of broken copper, or he gives rice of superior quality and receives a greater amount of inferior quality rice, or he gives gold that has been crafted [such as a piece of jewellery] and receives a greater amount of gold that has not been crafted, it is usury and unlawful.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling