Is it possible for PO approval hierarchy to see only Open PO amount based approval?

We have a scenario:

Only the dollar amount corresponding to open PO lines be counted when submitting to PO workflow.

Example: PO is $5 M , it has all the lines closed except 1 line. Now the outstanding line 1 is adjusted the amount and PO is submitted for approval. We want the workflow to see only outstanding line1 to trigger approval hierarchy not based on total PO amount.

Since all the lines are already received and closed, the new workflow should only be seeing outstanding amount. Is it possible, any ideas to make this work?

Appreciate your help.

Thanks
Raghu