How to enter self-billing invoices

If customer is making creditnotes/self-billing then you need issue invoices the same as for any other customer, but need to save also customer self-billing invoice number. in order to do it in customer form Invoice tab Check "Customer self-billing"

If checkbox "Customer self-billing" is checked then in this customers Outgoing invoice will be available Field Self-billing No which later when Customer will pay for invoices it will help recognize for which invoices payment is paid