If one of your customers has cancelled a booking that they've already paid for, then bookwhen will automatically show them as requiring a refund in the bookings list. If so, you'll notice the refund value is highlighted in red.
At the moment, bookwhen will not automatically refund a customer when this happens. This means you manually control when a customer receives a refund. If you'd like to perform a manual refund for a booking:
Click on the bookings tab.
Select the booking you'd like to action an invoice for by clicking on the booking code (e.g. WFT4G).
Now, on the right hand side click the View payment link.
You'll now be taken through to the website of your chosen payment provider. From here, log in if necessary and click the action for refund.