A payment installment plan is an agreement between you and the Illinois Department of Revenue to pay your tax delinquencies using a monthly payment plan. Vehicle Payment Plan Agreement.A vehicle payment plan agreement is a contract between a buyer and seller of a vehicle that agrees to installment payments. An Illinois motor vehicle bill of sale documents the purchase and sale of a motor vehicle between a private seller and buyer in the State of Illinois. The document should include the transaction date, purchase price, and a description of the item. Motor Vehicle (DMV) Bill of Sale Form Affirms the information contained in a transaction of a vehicle for a payment amount. Outline the payment terms, including the down payment (if any), installment payments, interest rates (if applicable), and the due dates. 5. An Illinois motor vehicle bill of sale legally transfers ownership of a car or other motor vehicle in the state of Illinois. Attached is the Installment Agreement Form you requested. Please complete the form and submit it to our office at the above address.