Whatever is in the heavens and whatever is in the earth is Allah's; and whether you manifest what is in your minds or hide it, Allah will call you to account according to it; then He will forgive whom He pleases and chastise whom He pleases, and Allah has power over all things.

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."