Buying and Selling: Selling on Credit to Someone Who Does Not Know the Price
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2118
If a person sells a commodity on credit to someone who does not know its price and the seller does not tell him the price, the transaction is invalid. However, if he sells the commodity for a higher price to someone who knows its immediate exchange transaction price – for example, he says, ‘The commodity I am selling to you on credit is £10 more than its immediate exchange transaction price’, and the buyer accepts, there is no problem.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling