@spur8thif the system is blocking you from renting the car, there isn't much you can do. Maybe speak to your manager and reserve the rental car outside of SAP Concur or call the travel agency directly to see if you can reserve the cheaper car. When the system blocks you from making a selection it is due to rules your company has put in place. I know they might not seem logical to you as an end user, but the rules are in place for some business reason.