Buying and Selling: Discovering a Defect in the Payment of Exchange
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2140
If a seller realises that there is a defect in the specified payment of exchange for the item that he sold, in the event that the defect was present before the transaction and he did not know about it, he can annul the transaction and return the payment of exchange to its owner. In the event that he is unable to return it due to a change in it or it having been utilised, he can claim back the difference in price between a non-defective and a defective item as per the instructions mentioned in the previous ruling.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling


