Currency VPRS in sales order

Hi experts,

I have this problem:

I have pricing in sales order with product costing( condition VPRS) and mark up;

when currency document isn't EURO, for exemple USD, the system reads the change .

If i modify the excange rate, the condition VPRS isn't correct(remains at last exchange)

exemple :

Currency document USD

Condition VPRS 10 Eur

exchange rate 1,5

VPRS in USD 15 USD

If I modify the exchange rate in 1,8 .. the value of VPRS in USD is always 1,5 instead of 1,8

Can you help me?