Invoice approvals at line level

Summary

Need line approvals for PO matched invoices

Content

Our company receives some invoices without the PO number clearly stated and we have been holding them outside of the system until a PO number is known and then we enter the invoice with the PO at the header level.  We would like to enter all invoices into our system for tracking whether a PO number is given or not.  Once we get the PO number, we will go back in and match the line to the correct PO/Receipt.  When I test this functionality, I am getting approval workflow errors.  Is this functionality available and able to be used in