i buy product one for 100 $
and product two for 100 Eur
 so i should wite the cost of each product with its original currency
 then i day that i will sell product one for 120$
 and product two for 120 Euro
 and in another place i should put the exchange rate for each currency
 product one price will be 120 $
 or i want to sell it with product 2
 it should be automaticaly 89.65 Euro
 accourding to the rate of the day