The amount to be paid on the spot and the currency of payment are indicated in your voucher. Moreover this information is available while booking a transfer. The payment should be made according to the official rate valid on the day on which the transfer service is provided and  in that country where the transfer is provided. What official rate will be valid in the future within the day of transfer service we do not know and it is not possible to forecast it. But it is possible to book a transfer with a 100% prepayment on the site, and then on the spot you will not worry about the currency exchange and its exchange rate.