You can't steal 600k and walk away. If they try it they will be sued for millions of dollars, and the guy will win.
They've stolen people's money in the past, so they will certainly try. Something Awful raised $30,000 for the Red Cross Katrina fund, and Paypal froze the assets, closed the account, and refunded all the money minus their fees! Any 'investigation' they claim they are doing is actually just consulting with their lawyers to see if they can pull this one off. The developer's only defence here is the make as many waves on the Internet as possible about this. It's much harder to steal something when everyone is watching you steal it.
Hopefully the guy gets his money back without needing to go to court, and hopefully any business still foolish enough to be using Paypal will think twice after hearing this. If Paypal has customers with $600,000 euros, then I think it's long overdue for the government steps in and regulate them. They are pretty much a bank without any of the rules real banks have to follow.