Hello @Anthony123,
You may have done this already but I'd suggest arranging the cancellation by contacting Airbnb and having them cancel the reservation. If you cancel it yourself you'll then have the extra hassle of contacting Airbnb to explain the extenuating circumstances and having them remove the "The reservation was cancelled by the host" review that's automatically added to your listing when you cancel. You'd also have to check that they remove the other applicable cancellation penalties should your circumstances qualify as extenuating according to their policy.
Contacting them first will also give you certainty as to wheather the circumstances qualify as one of their official extenuating circumstances.
As far as I understand it there isn't a way to offer a refund until the guest's checkout day.
Contact-Airbnb-A-Community-Help-Guide
Extenuating circumstances policy
Host cancellation penalties