Bookings can be cancelled anytime (by both renter and owner) as long as the payment hasn't been compeleted by the renter.
In case a booking already has been paid, a booking agreement is automatically created between the owner and renter. This also includes cancellation conditions that the owner and renter must comply with in principle. Download this agreement from your 'bookings' overview and click on 'view' to go to the rental agreement.
If you, as the renter, want to (partially) refund the renter as a gesture, you can simply leave the booking as it is (you will also receive the earnings from Gearbooker after the end of the rental period) and pay back the amount to the renter. This can be done outside of Gearbooker.