MAKE due allowance for man's nature, and enjoin the doing of what is right; and leave alone all those who choose to remain ignorant.
Al-A'raf (The Heights) Verse 199
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