>> Though I don't know the exact technical points as I am not with that customer anymore. With the below information and a bit of debugging, you can get it done.
>> We enhanced the option Create Invoice->For a Purchase Order & Changed the Controller class to have a RFC call to ECC to find out the line items and add it up to the internal table.
>> ECC Function module which we developed has the logic to decide when and under what conditions the line items to be populated.
>> When a PO is replicated into SUS - we enabled a flag to allow Invoice Creation against PO using the XML inbound BADI. You can find this field in BBP_PD in the HEADER details & search for "Invoice".