If the Currency Code is being automatically populated, it is likely being selected via a stored procedure. To confirm this:
Log into FocalPoint Administrator
Browse to System Options, Purchasing > Purchase Requests Setup or Invoice Register Options
Highlight the affected invoice type and click Edit
Click on the Header tab
Identify whether the Currency Code is set to come from Stored Procedure
If the Currency Code is set to come from a Stored Procedure and is coming through incorrectly, the Stored Procedure is likely going to need to be looked at and changed.
The name of the Stored Procedure will be TSSP_GET_PR_COST_PRICE_X - where X is the Sales Request Record ID, found in the blue bar at the top of the screen when editing the Purchase Request type (i.e. after Step 3 above).
Once you have the name of the Stored Procedure, you will need to look at it in SQL Management Studio and correct as required.
Remember that if the Stored Procedure is very complex or this takes you a significant amount of time, it may need to be referred to consultancy.