@Robert74, I suggest you contact AirBnB straightaway and tell them what you've just told us.
See what they have to say about it, especially the fact the guest was trying to blackmail you with regard to his review.
For future reference, never, ever offer to refund cash to a guest. Strictly unnecessary and against ABB policy.
And never feel intimidated by the threat of a bad review. Remember, you can always respond to it.
Meanwhile, write an honest review of your guest's visit. If there was anything positive, mention it. But don't neglect to say that he tried to pull the old 'refund for a good review' bluff on you. Other hosts need to know what this guest might try on them as well.