In default Odoo it only possible to add a customer reference or a invoice number. This module combine these to in [customer ref] / [invoice number]. Setting needs to be done on journal level, those over there het option "Customer Reference with Invoice number"

How does this module work?

Step 1) Choose on Journal level the option: "Customer Reference with Invoice number"

Step 2) Confirm the invoice

Step 3) In field payment_reference (in tab info) you will see now customer ref / invoice number