To use this module, you need to:

1.  Go to "Invoicing -\> customers -\> Invoices", create an Invoice and
    validate it by clicking on the 'Confirm' button.
2.  Create a Credit Note by clicking on the 'Add Credit Note' button.
3.  In the form view of that Credit Note you can see the new field
    'Invoice reference' that is a link to the Original Invoice (the one
    you created in step 1)
4.  Come back to the original Invoice created in step 1 and you will see
    a new page in the notebook called 'Refunds'. There will be the
    Credit Note that you created in the previous step.