Briefcase allows you to change the generated invoice number.
Mac
To modify the invoice number, Navigate to the invoice you need and whilst holding down the option/alt ⌥ key click on the invoice number. Once the dialog appears simply type in the new purchase number and press done.
Windows
To modify the invoice number, Navigate to the invoice you need and whilst holding down the alt key click on the purchase number. Once the dialog appears simply type in the new invoice number and press done.