@ Joylyn:
I agree with others to the extent that rather than reducing the salary, a better option would be to issue a formal memo to the employee (preferably quoting the section of the employment agreement or appointment letter dealing with genuineness of employee-given information) stating how he/she has not complied with the employment policies and hence he/she would be liable to be terminated/demoted or any other action as the company policy prescribes. If you wish to retain the employee but at a lower rank and salary, you could add something to this effect:
"However, in view of the employee's good conduct and performance, the company has decided to retain the services but at a (lower) salary and (lower) position."
@ kennethnyc1:
I am not sure if you can reduce the salary of an employee for one mistake which cost the company $500, as long as the person is qualified to be in the position he is in, as there is already an agreement between you and him on a particular salary amount. You could issue a show-cause notice asking the employee for an explanation for his conduct which has resulted in the losses, and then based on a formal discussion with the employee, implement any salary cuts (of course, all of these have to be documented, with reasons).
Your friendly Lexorcist