Official Letter No. 3711/TCT-CS dated September 08th, 2020 issued by General Department of Taxation about Guidance on issuing invoices for providing services.
According to the regulations, issuing date for providing service is the date of completion of the service regardless whether the money has been collected or not. In case goods are delivered several times or a completed item or stage is transferred at a time, an invoice shall be issued for each time.