@Helen18
35 nights is a "long term stay" reservation, having the "long term stay cancellation policy" attached. If you cancel as a host, then you will be paid the nights the guest has stayed.
In your case (as you describe it) it will be at least approx the first installment payed to you.
If the guest cancels, max. 30 nights after the cancellation date will not be refunded.
Instead of cancelling a stay, also shortening a stay ("change" option) can be an option and is in most cases a more flexible tool.
it is not clear to me why the reservation is cancelled and why Airbnb refunded the guest in full. Something unuseal happened during the stay ?