What did you save onto an MP4 player? The wallet.dat? The seed phrase? And why did you do that on an MP4 player? How about providing any kind of proof. Pictures of the MP4 player, proof of purchase, bank statements, etc.
There was no seed at the time, so it's either private keys or wallet.dat - but the OP didn't mention anything about how the purchase was made, or what exactly he saved - so maybe he only saved his BTC address and doesn't actually have anything of value.
To be fair, I believe this to be a scam. Nobody would be dumb enough to pay you $500k for a potential chance of recovering the BTC from the flash memory.
It seems that way, because is there an easier way to make $500k with no obligations other than maybe sending someone a broken mp4? Apart from that, I would think differently if I were in a similar situation. There are data recovery services that, for a fee, allow the client to be present during the data recovery process, so I advise the OP, if he is telling the truth, to look for one and try to save his lost treasure.