Employment Law Questions? Ask an Employment Lawyer.
what state are you in?
In Texas, employees are presumed to be "at will." At-will employees may be terminated for any reason, so long as it's not illegal
Here, you dont need to justify the reason but keep in mind practically that failure to return the money may be a matter of innocent mistake rather than intent to steal. That said, you can terminate for any reason and the exceptions to that general rule do not apply here.