Is it permissible to reward an employee who makes an extra effort outside his working hours to find cars for me to buy at the same price as the parent company, knowing that he does not ask for anything in return?

There is no harm in an employee being given money for a service he performs outside his working hours, as long as the employer is aware of it, and that service is not part of the employee's core duties for which he receives a salary. This is not considered among the gifts given to workers, nor does it involve harm or a violation of employment terms.