Q. Can one exchange currency on a rate other than the market exchange rate?
A. It is permissible to exchange one foreign currency in lieu of another foreign currency based on any rate agreed upon at the time of the exchange.
Thus, if at the time of the exchange, the buyer and seller agreed to change for example, dollars in lieu of pounds on a rate lower than the market rate or higher than the market rate, it would be permissible. This would not be counted as Riba/interest.
Allah Ta’ala Knows Best
Mufti Ismaeel Bassa