The sale of money for money (currency exchange) or what is known as "sarf" is governed by precise Islamic regulations to avoid usury. The basis for this is the prophetic hadith: "Gold for gold, silver for silver... equal for equal, hand to hand. If these types differ, then sell them however you wish, as long as it is hand to hand." (Narrated by Muslim). To clarify, its sale is permitted only for collection purposes and not for the exchange of money for money.