You could pull an arbitrary ancient broken HD from the waste and dump it from 50 metres, finally claiming that there was 300 BTC on it.
As long as you can't recover the data, you can't proof that the 300 BTC wasn't there.
You could prove it by looking through all addresses that haven't been used since the date the HDD is claimed to have started malfunctioning (all addresses that had a balance by then but didn't have any outgoing Tx).
That would leave you with a sample of addresses. Now you look through them for addresses that contained 300 BTC at that date. If there are none, you have proven that it was a lie. Also, if there addresses which have the exact amount within that sample, you just have to monitor them. If they spend something, you can strike that address and if there is none left, then someone didn't tell the truth!
Sounds interesting, On what website can you do that?
Ontopic: It a risky to buy the hd, but if the 50 bitcoins are on that thing its worth your investment.