It is possible to issue a partial refund:
1. Click on the booking to open it
2. Go to the Payments tab
3. Click on the Refund [1] button for the payment that you want to partially refund
4. By default, the system will show the amount paid. Edit the Amount [2] to be refunded
5. Enter the Reason [3] and a comment [4] (optional)
4. Save