I got a request like this for 30 days and I was kind of naive and stupid, and going broke, so I contacted the guy. He wanted to split the payout 80-20. It was less than what I would make normally in a month so I wasn't really interested, but I let him complete the booking. After he completed the booking I looked up the message template and then I realized it was a scam. I told him that I will not pay him until I get my own payout first, he was annoyed but he agreed. After I got the payout I contacted Airbnb, the police, and my bank to let them know of what was happening. Airbnb was really helpful in my case, and the police were too. In the end, Airbnb told me that the payout was legitimate and I should not refund anyone outside of their platform, and the police backed up their claim and said the money is legal and the guest has no footing to receive a refund. So I called the guy out on his scam and told him he can cancel the booking through Airbnb as I won't be sending anything outside of Airbnb. He freaked out and threatened to call the police, which I answered with a case number and a phone number he can call.
In the end, I got to keep the full payout, the bank advised I hold onto it for 6 months just in case, but after that, I was allowed to keep it.
So long story short, if a guest makes this kind of request and books through Airbnb, they have no power to actually demand any money back outside of the platform.