Buying and Selling: Selling Gold for Silver or Silver for Gold
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 2130
If gold is sold for silver or silver is sold for gold in an immediate exchange transaction, the transaction is valid and it is not necessary for their weight to be the same. However, if the transaction has a period, it is invalid.
-Ayatullah Sistani, Practical Laws of Islam, Buying and Selling