It would be a laugh if it turns out the treasure chest itself was very valuable but worth nothing now because they broke it by forcing it open.
What is the protocol when you find a chest or some other kind of container that is locked, are you meant to force it open/break the lock to get into it or should you report it first?
Items used in Detectorists telly series auctioned to raise funds for new village hall: https://www.bbc.co.uk/news/uk-england-suffolk-61105023
Theft is a police matter. However if somebody agrees a deal with a farmer to split a find 50/50 and reneges on the deal recourse is by civil action. Advantageous to the farmer because he only needs to prove balance of probability.
He didn’t “renege on the deal”, he lied about where he found it. It’s theft. I expect your sort who quite openly boasts on here about fraudulent claims and ripping people off thinks that’s fair game.
And that says it all really. Challenged with the evidence and an explanation of the legal status of such a deal you fall back on nothing more than an inanity. How about that wager?