You paid by the payment books so go by that. Forget what the new VP says, You followed the written book payment plan and you do not have to go with another oral agreement this new VP wants to foist on you for an added $2500
Contact a consumer protection attorney and have them review your documents. Most will provide a free consultation, and many will take good cases on a contingency basis. A good place to find one if the Chicago Bar Association's attorney referral service or the National Association of Consumer Attorney's web site: naca.net. There are some legal arguments based on estoppel that might work in your situation. Also, the Fair Debt Collection Practices Act will apply to the collection agency, and if they have reported anything derogatory on your credit, you may be able to set up a Fair Debt Collection Practices Act case by disputing the items with the credit bureaus. A consumer protection attorney can advise you on the best way to proceed using various consumer protection statutes.