One contract is the car purchase or sales agreement where you contract with the dealer to purchase the vehicle. The final hurdle you face is signing the vehicle purchase agreement; a confusing and convoluted document that we explain in detail here.My bank wants purchase agreement or bill of sale before sending out a check to the dealer. When you buy a car at the dealership, you'll need to sign the car purchase or sale agreement along with other documents. Find out what you'll sign and why. In this guide on how to buy a car in Texas, you'll learn what documents you need, how to get financing, and more for new or used cars. Watch this before you finance your next car (former dealer explains how to read a purchase contract) The retail installment contracts in the list below have been accepted as plain language contracts under the provisions of the Texas Finance Code.