How do I dispute this?

You need to call the person that sent you the letter and find out why there is a difference in the balance and tell them that you have a letter from a week ago that says one amount and now the new letter says a much larger amount. Do you have the judgment paperwork that states what amount and how much interest and penalty's you are to pay? If they fail to help you then take all that paperwork to the court that the judgment was placed in and talk to them.