Dear TW:
For the normal process in our company we create sales order --> Outbound delivery with materials with price TAN and material free of Charge, so for regulation is not allowed to get and invoice without value, and the user only create the outbound delivery for the free of charge item, that´s the reason we add a validation in VF01 and VF04 to restrict save and invoice with zero value.
And the user exit was updated after the accounting document is created. So later when we update the user exit now the invoice and the accounting document was not created.
Regards Antonio