A:
If the currencies are of the same country, they cannot be sold at a rate different from their face value. However, if the currencies are of different countries, they can be sold on spot at whatever rate agreed upon between the parties which can be different from the market rate. However, if the payment is deferred on either side, it must be in accordance with the market rate. This condition is put to restrict the use of this transaction to the genuine needs, otherwise it may be taken as a device to effect riba transaction.
Contemporary Fatawa
Mufti Taqi Usmani