Wow that is a lot to swallow, no less handle! I am very sorry for your struggles and hope relief is coming soon. I have a question, is there any reason you are unable to take your children with you? Just a question - no judgment.
As far as a letter, I would see if there were legal aid in your county to view it prior to having him sign. The last thing you need is a legal issue. Maybe google an example if you are unable to get free advice yet make sure you cover your butt and protect your children as much as possible.