You can change a system parameter so that any items added to a purchase invoice or any changes to the purchase invoice will use the current rate from the system. You need to be sure that the latter is what you need as well and that all your other users are out and you have user permissions to do this before you change this setting.
Before changing in your live system, test in a copy company to make sure, that this change meets your needs and works how you expect
Select the System tab on the main menu
Then Application Configuration, System Parameters
Click on Purchasing
Press Save to take you to Page 2
In section When adding items to a purchase invoice or changing items on a purchase invoice which exchange rate should be used in the box next to that enter 2
Save 2 more times then ESC
Select Yes on ok to amend
