You seem to be making an awful lot of excuses to not just pay someone who brought to light a critical exploit. Do you work on the security team or are you a lawyer (maybe with a panicking accountant looking over your shoulder) trying to find fine print reasons say, "Aha! We can save money to our bottom line in this instance!" ? Do you know how silly it looks for you to make these excuses?
That is a vague reply with no apparent relevance to anything I said except the amount $500 which was not to be taken literally. My point was that facebook isn't doing this to save money and believing so is idiotic.