Uhmmmm, I think asking for permission might be in order. A mean, his talking about excavation and probably that would be him altering the site and possibly, the suspension of further dumping of dirt on that site.
Again, its a really serious one from what's his offering and I'm also supposing he doesn't have the resources to get the work going and would need the help of his city in funding the process too. A process that could be said to be a high stake probability of success looking back to the time this hard drive was dumped (8years+). So much for a rash action!
I've read an article from last year about this guy and there was a company that offered to excavate for him. They have heavy equipment and were ready to strike a deal. They'd look for free and if they were to find it he'd give them half of the coins. I feel like the biggest problem here is all that red tape.