Am I sinful, and does my writing of promissory notes put me in the category of one who writes usury, especially after I discovered that the amount for which the car was purchased in installments was originally an interest-bearing loan?

If a customer borrows an amount to return it with an increase, this is usury (riba), and the one who practices usury, its scribe, and its witnesses are cursed. The scribe of usury is sinful if he is knowledgeable and intentionally writes down a usurious contract; otherwise, there is no sin upon him.