Buying and Selling: Selling Gold for Gold or Silver for Silver
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2129
If gold is sold for gold or silver is sold for silver, irrespective of whether the gold and silver are minted coins or not, then in the event that the weight of one of them is more than the weight of the other, the transaction is unlawful and invalid.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling