Buying and Selling: Paying with Unlawful Wealth
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2074
If a buyer purchases an item undertaking to pay for it later, but he wishes to pay for it with unlawful wealth, the transaction is valid. However, he must pay the amount he owes from lawful wealth to be absolved of his responsibility [to pay the seller].
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling