Flying is such a far way away from what it used to be. Enforce nude tubes, fondling, nasty governmental and airline employees, and on on. United has some of the worst of the airline employees but...
$800 is the maximum allowed per the government regulation, they handled it according to their contract of carriage which is the contract between the customer and united, he disobeyed crew instructions, and then resisted (the last two are chargeable). He will not have grounds for a lawsuit. United adhered to their contract of carriage and appropriate regulations. This guy seems like he was unhinged considering he then tried to run back on the plane. I’m not saying it’s right it’s actually pretty terrible but that’s flying in the USA for you.
I imagine United will give him something because of the PR damage but he has no grounds for a lawsuit. The damage will come from the story, not from a lawsuit.
Maybe the guy, as he was at the door, heard ‘I’ll take the voucher!’ and ran back because he *really* needed to be in the office at 7am today.