Andrew,
I was testing your scenario but need more information. Please confirm the following:
1. Create a Job to a Customer who has your Local Currency (i.e. USD)
2. Setup a Task and Budget&Billable planning line that is for an Item that you need to create a PO for. Set the cost and price
3. Create a PO for the Job Item to a Vendor in another currency (i.e. EUR). Receive PO in 1 Month and Post Invoice in next Month
4. Job Invoice Customer for the Job Item Line, LCY Currency and for the Unit Price set on the Planning Line.
Please walk me through your example and what is resulting.
Thanks,
Steve