Verify the information on Final/Invoice view.
All all Material from Time Rerpots to the Project through Project Materials. Then remove all Materials from Time Reports, Medi Orders, and Project Work Orders (if in use). To remove it from the Media Order/Project Work Order it needs to be opened first.
Then as a Producer you can inform Accounting that the project is ready to invoice.
Accounting can Part Invoice the project and only select Materials!
Next step is to close the Project (Active > NO) and create the final invoice on project: Marke Project & choose: Create Invoices (Selection: 1).
Final Invoice doesn't contain Materials but everything else.