Please follow steps below to create invoice with custom invoice number.

Step 1 : Click on Sale Invoice menu.

Step 2 : Click on Add new button.

Step 3 : In invoice detail section find Invoice No column & add new invoice number.(Will not accept used invoice numbers.)

Fill up the details of customer, products & payment details. Click on the Save button given below.
Note: When you create the next invoice it will auto-check the last used invoice number (Ex: 444) & will show you the next number automatically (Ex: 445).