Two methods of guaranteeing an exchange rate are the fixed rate and the forward contract.
Fixed rate is the process of exchanging currency at a rate set by reaching an agreement with the foreign vendor as to what exchange rate will be used. Both the vendor and the buyer assume some risk that the spot rate will change in the other party's favor.
Forward Contracts are used by companies who make large or frequent purchases from foreign suppliers. The seller (typically a bank) assumes the exchange rate risk for a fee.