EXCHANGE RATE CONFIGURATION

Hello, I have set our companies base currency as mexican peso. We charge services in mexican pesos to some clients and US dollars to other clients. There are two different price lists, one is not a calculation of the other. I have programmed the customers with their assigned currency and this works fine. It shows the USdollar on the us client and the MXN peso on the mexican client. But I see that when I add items to the catalog I cant assign them different currencies. For the program to truly manage multiple currencies I must be able to assign the item its currency and then when the sales are calculated the exchange rate at the time of the sale of the item should be captured to accumulate the correct sales in the base currency. How do I capture different price lists in different currencies and assign them to the customer?