BELIEVE in God and His Apostle, and spend on others out of that of which He has made you trustees: or, those of you who have attained to faith and who spend freely [in God’s cause] shall have a great reward.
Al-Hadid (Iron) Verse 7
Allah's Apostle said, "Don't sell gold for gold unless equal in weight, nor silver for silver unless equal in weight, but you could sell gold for silver or silver for gold as you like."
Sahih Bukhari: Volume 3, Book 34, Number 383