Update convertion rate as an admin.

Available for admins who has "execute_cancel_pending_transfer_requests" permission.

NOTE:

• For cross-currency transfers there is an additional parameter “Exchange Rate”.
• While the request still has “Pending” status, the administrator can manually change the value of the “Exchange Rate” and update the request.
• All amounts in all transactions included into the request are recalculated according to the updated exchange rate.