AP Invoice:Match to receipt: unbilled/overbilled/underbilled logic

I am working on automating invoices from vendors to AP. . Need some help in deriving po line number.

Summary:

We have very few key information passed on to us to work with. Information comes to us in tab separated format.

here is what our vendors can send back to us.

VENDOR - constant

Invoice Date

Invoice #    --> this is inv number unique to their system

Vendor Part Number   --> they pass their internal part number. they were unable to send us oracle part number that we send them in printed PO.

Description --> line description

Qty Invoiced  --> quantity invoiced , could be more or less from the ordered quantity.