How to generate an ERS invoice from an ERS Puchase Order which is already closed

Hers' the issue :

- We have an ERS PO. Both the PO Header and all 4 PO Lines are closed.

- Qty Ordered = Qty Billed = Qty Received on all PO LInes.

- All PO lInes have Amount Received = Amount Billed  EXCCEPT for PO  Line 2 where  Amount Received ( 3,000.00 ) and Amount Billed ( 2,000.00 ) . This is a difference of 1,000.00 which hasn't been paid to supplier yet.

- How can we generate an invoice based on the above information, given that this is an ERS PO and Qty Received are completed for all PO Lines.

- Only issue is that for PO Line 2 , the Amount Billed is less by 1, 000.00. I assume that user went to change the Unit price after the PO LIne has been received.