Buying and Selling: Receiving an Extra Item or Imposing a Condition (Usury)
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2081
If the extra thing that a seller receives is different to what he sells – for example, he sells 1 kilogram of wheat for 1 kilogram of wheat and 10 pence – it is still usury and unlawful. In fact, even if the seller does not receive any extra goods but makes it a condition that the buyer must do something for him, it is also usury and unlawful.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling