Definitely request a revision and do not sign until/unless you're happy with the amount on the paperwork. What they've given you has you liable for the full amount--there's no settlement. Whatever you sign to is what you will be obligated to, regardless of prior discussions, negotiations, offers, etc. Ask the law firm to revise the amount in line with the letter you have and your discussions. If they won't, there would not seem to be incentive for you to sign or accept the agreement; you would not actually be "settling" the debt but instead you would be agreeing to pay it in full. What they've offered you is basically what you'd have  to pay if they sued you and won.