If a purchase is not assigned to an invoice when it is set up, the option to add an invoice number does not appear on the expense details page.
To add an invoice number to an unassigned purchase, go to purchasing, find the desired supplier and open the Supplier Details page.
Click on the Unassigned Parts tab
In this tab, select the part or parts you wish to assign to an invoice:
and type the appropriate invoice code (number) to the text box on the right of the page as shown below: