Buying and Selling: Adding an Extra Item to Avoid Usury
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2082
If a person gives a lesser amount but adds something else – for example, he sells 1 kilogram of wheat and one handkerchief for 1.5 kilograms of wheat – there is no problem as long as he intends the handkerchief to be the item for which he is receiving the extra amount [i.e. the extra half kilogram of wheat,] and as long as the transaction is an immediate exchange (naqd) transaction.[1] Similarly, there is no problem if both sides add something extra – for example, one of them sells 1 kilogram of wheat and one handkerchief to the other person for 1.5 kilograms of wheat and one handkerchief – as long as they intend the handkerchief on the one side, and the handkerchief and half kilogram of wheat on the other, to be the items of exchange.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling
That is, a transaction in which there is no lapse of time between the buyer paying for the item and receiving it. This is in contrast to credit (nasīʾah) and prepayment (salaf) transactions.