It's a classic ethical problem. The miner gains nothing and loses everything from whole ordeal. 500k is 500k after all and it's not like you could find a loose 500k when you walk out your door for that morning jog or to grab a bite for breakfast. The miner has to give the money back to the user out of his own pure moral compass and all that. Which I myself don't think is a great basis anyway considering the implications of todays' world setting. In any case, I still think the miner should return the fee since that's still someone else's money. But we can't be very hopeful really, since the miner's anonymous, and we don't have any way whatsoever of rendering him accountable should he run off with the money.