Exchange rate determination at time of Good Reciept

My client has a manufacturing operations in Venezuela. They regularly import production materials as well as supplies etc. to run the business. The Venezuelan government defines the exchange rate on different types of purchases against the US Dollar.

When the goods are purchased the exchange rate is not known. The Exchange rate has to be applied to the transaction at the time of goods receipt. The exchange rate will be different for different types of purchases. For example: Raw materials are 2.6 Bs per dollar, Supplies are 4.3 Bs per dollar and all other purchases are at the rate of 5.3 Bs per dollar. The exchange rate must be entered correctly at the time of entering the Goods Receipt.

The exchange rates are controlled by the government and stay static for a certain amount of time, but can be changed with little notice by the Organization in control.

Has anyone on a global SAP implementation or who has worked locally in Venezuela encountered this in the past?

If so, how have you handled it in the system?