I don't think emailing him would be dangerous as long as you don't open any attachments from him, but admittedly I have no idea what a good hacker can do with an email. Imo he was probably just trying to have some fun/display his l337 hax0r skills. I would send him an email congratulating him and telling him he was the first to get past what you thought was a very secure firewall. Ask him how you can fix it and he might even give you security tips. Stroke his ego a little bit and you might get what you want. That being said, if he asks for money or starts playing games I wouldn't negotiate.