There is no cancellation notice showing in your reviews, @Christopher1189 , which would say to me that Airbnb was on your side with this one.
And, no, you don't owe an Airbnb guest a cheap stay in a more expensive place. That is a misconception by some guests: that because your less-expensive place is unavailable, you somehow now owe them that price somewhere else.
I'm with @Helen3. I can see why the guest is upset. I've been in their shoes -- you think your accommodations are all sorted, then you have to start from scratch for a flimsy-sounding excuse like "I haven't had a chance to check the property." It's disheartening.
But you know that, and will have tried everything possible to make the stay happen. We have see-sawing regulations here, too, adding layers of difficulty.
If the guest persists about the money and gets belligerent, you may have to block them. I'm not sure about the blocking procedures these days, so it will be something to look up.
One thing to know is that Airbnb likely provided the guest with a bit of a credit for their trouble. So they will have applied that to their new booking. Sounds like it wasn't enough to cover the difference of course. But you don't have to cover the difference, either.