Tough, not your fault. Many guests think Airbnb operates like a hotel. That is, if they leave early they don´t need to pay for it. Unfortunately, unlike a hotel, it is virtually impossible to re-book the lost night(s) at last minute. At the end of the day, the guests booked and fully understood the cancellation policy (if they bothered to check prior). I would not do anything and refer them to speak with Airbnb helpdesk. Let airbnb sort it out. If the guests leaves a bad review, you can do a follow up comment saying that due to the last minute departure, you were not able to refund in accordance with the cancellation policy. Most future guests reading the review could understand that. Giving in to blackmail is never a good thing and who knows really if the excuse is real. Sorry.