You dont need to make a new post muddle, you can add comments in your original post. If that is the case it should be fairly straightforward to find out when the "advance" was made and in what form.
It is something that has to be sorted out between the employer and employee. Nobody on here would be able to comment who is right or wrong I am afraid.
whilst taking the money back may be illegal (depending on circumstance - more info needed) it is NOT an offense and the police have no power to investigate it