Let me explain bit clear about your requirement :
You are sending component A and B to vendor Z, you are purchasing the third component C from the same vendor and then finish goods D become ready with the component A,B,C.
Now you have to pay the vendor for the purchase material C and the job charges for the finish goods.
Am I right ?
Before invoice, can you explain that how you are mapping the scenario in your SAP system.
Are you creating a standard PO for the component C ?