I suggest you start by asking that lawyer how he justifies the $5000 when your account shows $1800. Though I suspect that you aren't talking to the lawyer but to some flunky whose job it is to hassle you.

If you can actually talk to a decision maker you might start by offering the $1800 plus maybe another $1000 to cover the debt and a reasonable amount for his time. Then insist on a personal meeting where he will hand you the signed settlement agreement in exchange for your check.