Hey
The exchange rate on the Billing header will be the exchange rate that is given for that particular day on OB08. If you give manually the exchange rate in sales order, then it will pick that exchange rate for billing header as well.
So if your header exchange rate is taken from OB08 then you can configure to pick the exchange rate for the item as well on the billing date. This can be done through copy control settings. Following options are there for the item exchange rate determination.
A Copy from sales order
B Price exchange rate = Accouting rate
C Exchange rate determination according to billing date
D Exchange rate determination according to pricing date
E Exchange rate determination according to current date
F Exch.rate determination accord.to date of services rendered
Regards
Hussain