I did this for a client who had to bill one specific customer in Canadian dollars. Created a UDF in the Customer file and instructed them to put in an exchange rate. Then I modified the Invoice Printing form by creating my own formulas for the Unit Price and Extended Price to use the value in the UDF to convert if the customer number matched. Then changed the totals and it worked fine. When the customer paid, the bank converted back to USD and they posted that amount to A/R and the balance to Gain/Loss on Foreign Exchange. Not a great solution if it needed to be used for a lot of customers or a large portion of the business, but it did solve the issue. Before I did this for them, they were entering the order by overriding each price to the Canadian Price (from the customer PO), and then after printing the invoice, they would change all the prices back to USD before updating...when they remembered to.