A requirement of a valid contract is including the essential terms.  The essential terms of a contract are identification of the parties, subject matter, price, and time for performance.Since the contract identifies the parties (your dad and the company), the subject matter (chassis), price ($25,000), time for performance (60 months of payments at $497.56 per month), the contract signed by your dad is valid.The fact that it did not include the total with interest does not void the contract because the essential terms of payment and price of the chassis were included.   The remedy here is reformation (correction / amendment) of the contract to include the total with interest.