Implementing "Future currency" in NAV?

Have anyone implemented "Currency Future" in NAV?

"Currency Future" in short: Sales Invoice sent to customer today in Foreign currency. Due date set six month after. To reduce currency risks you can get a contract with your bank to lock an exchange rate for your invoice amount to an agreed rate to be used the on the due date of the invoice.

Longer description see:
http://en.wikipedia.org/wiki/Currency_future

Problems I see with this in NAV is that I get a certain exchange rate for a specific sales Invoice that is not bound to current date. In NAV standard I can have one exchangerate per day but since i can have several Sales invoices with different Exchange rates on the same date (More than one Sales invoice with the same due date).

(I will have the same scenario for purchase invoices but if I find solution for sales i can probably use the same for Purchase)

Br

Martin Jinnelöv, Sweden