Well this is a classic example of how to sue someone for slander.  First of all, what you stated is not true.  Second of all I warned you not to slander us.  Now you did it and my attorney will begin a suit against you tomorrow.  You in fact were paid in full and you did in fact over charge us by billing us for more than you agreed in your engagement package.  You are now angry because I caught on and you didnt get paid for what you overcharged.  See you in court.