Third-Party Sales order item - Partially invoiced after full IR on MM

Hi

I have read relevant discussions on SCN and checked note 210500 but still cannot understand my situation.

We are having standard SAP third-part sales process - item category TAS

1. Thrid-party sales order item S1 - 100 PC

2. PO item P1 created - 100 PC

3. Somehow the sales order item S1 has to be split into 60 PC + 40 PC - so finally S1 (60PC) + S2 (40PC)

4. PO item P1 (60PC) and P2 (40PC)

5. Vendor invoices coming booked for P1 and P2 with correct quantities 60 and 40 PC

6. S1 and S2 customer invoices issued with sames quantities 60 and 40 PC

7. Now S2 status is fully invoiced.

8. S1 is still with status Partially invoiced - if I call VF04, I can create one more billing for 40 PC against S1

What was wrong and how can I fix it (I don't want to mark the line S1 for rejection because it will impact my report)?

Any input would be highly appreciated.

yu