There is no hack in the bitcoin client that allows you to double your money, it just doesn't exist its just a simple ponzi or scam from the jump, would totally stay away from this site.
Not to defend anyone, I know this is a scam, but I think, for clarity sake that the guy is saying that he found a Ponzi and he has hacked it to double his coins in seconds rather than the 72 hours or whatever and he wants us to pay him so he can double our money and send it back, apparently with no fee or anything, not even the network fees(lol).
Even if you could hack a site like that, the available balance is traditionally just not there. If the first 100 put in 0.001, then that is 1 BTC in the warm wallet. That would stay there for as long as 72 hours or whatever. So the max you could put in and get right back out would be 0.5, and then get back that entire 1 BTC. That is possible, but you are also dependent on new deposits, just like the Ponzi site itself needs new deposits. You would essentially cause the site to fail before it's normal life cycle of a week or two. If you move forward from the moment of the OP, the site would have been killed by this one guy and be gone. A good Ponzi admin would also see the missing funds and run. Also, this guy mentioned 41 BTC in the first few hours of the site. If the max deposit is 1 BTC and the minimum is 0.001, then the average deposit would be around 0.01. That would mean around 4,100 people jumped on the site within the first hours or a bare minimum or 2,000 very stupid users in the same period.
Last possible thing about this mystery site would be the Ponzi admin backing the site ahead of time with 30-40 BTC. Why would a scammer do that? It makes no sense, with that much BTC he could legitimately do a lot. He could give out loans on here or his own site, invest in various things, that is like having $10,000 to start a small business and investing it all in a three card monte stand.
POINT IS....even if this guy could hack the site, and the funds were there, they are gone now and so is the site admin/owner. No smart scammer or real business person is going to keep running a site that just lost $18,000. If the site stated open, he or she would have paid a pro to find out where the hole was and plug it and the site would be down for maintenance in the mean time. So, just to be a nice guy, I will say this OP was never lying and stole 41 BTC from a Ponzi site. NICE JOB. But do not expect us to believe that the process is repeatable and still works at this time. Close up your thread and invest those funds.