A vehicle payment plan agreement is a contract between a buyer and seller of a vehicle that agrees to installment payments. I left with a signed purchase agreement from the dealership.A confusing and convoluted document that outlines the selling price, fees, taxes, your trade in, and more. We've put together this guide to simplify car purchase agreements and help you stay protected when purchasing vehicles or selling them. Not all states have an agreement with New York. Use this car purchase agreement template to customize the financing information for your car along with other required details. To draft a legal payment agreement for a vehicle sale, it's essential to include specific details and terms to protect both the buyer and the seller.