Is it permissible for me to exchange foreign currencies at a rate lower than the prevailing market rate, taking into account the distance of exchange shops, and is this transaction considered exploitation or not?

For the exchange of one currency for another to be valid, immediate possession must take place at the contracting session. Equality [in amount] or adherence to a specific price is not required, as what matters is the mutual consent of the contracting parties. There is no limit to profit beyond which it becomes forbidden.