I had a situation similar. I have my place listed on Airbnb and HA. I received and instant booking for a weekend thru HA..once I got home I received an update from Airbnb app and text that I had an instant booking for the same dates. Apparently I was in a no service area and it didn’t go thru right away.
So I was seriously stressed. I reached out to the person who booked (Airbnb) and told them the situation. Since I didn’t have my calendars linked (long story but I tried). I basically had a double booking.
Didnt get a response so I called Airbnb. Explained that I was really sorry and I had to cancel even if it meant I had to pay the penalty. Customer support was great. They contacted the person and since the deposit was still pending they were able to cancel them for me without penalty.
So I suggest reaching out to the guest first. Explain your situation, offer them a discount or another stay. You can send a different date/price over and if they accept that it lets you off the hook for the current day they booked. Otherwise I’d call Airbnb.