First, you must disable auto-binding.
When you click on the create invoice button, you create a new invoice in the database. - “Create a new thing” of the Invoice Item type, with all its fields.
In a second workflow, you must change - “Make change to thing” the result of step 1, linking this invoice item created with the other table of the Invoice type.
I hope this was clear to you and that it may have helped you in some way.