If you don't have all the details of an invoice to hand or you need invoices verified by someone senior before they can be approved, you can choose to save invoices as draft on the create invoice page.
This guide explains how to edit, approve or delete draft invoices.
Draft Invoice List
Navigate to Sales > Invoices > Draft to view a list of all your draft invoices.
Approve a draft Invoice
On the draft invoice list, click on the check box beside the appropriate invoice or click on the check box at the top of the first column to select all drafts and hit the Approve button.
Edit or Delete draft Invoice
On the draft invoice list, click on the appropriate draft invoice number and use the Edit button or Delete draft button as necessary.