@Hanna28
It depends on what you do....
If a guest makes a change to their booking before they arrive and start their reservation then the total that the guest pays AirBnB, including the booking fee, will reflect any change that is made. AirBnB does not take a second booking fee for processing a refund on its platform.
If the guest is staying with you, and you have agreed that if they only stay for one night, and you will refund them for the second night. Then you can use the resolution centre to send them a specific amount of money. There will be no change to the booking fee they have already paid AirBnB.