Business Conflict of interest?

I work part time at a small cancer research company. I mainly file papers and do a lot of grunt work. Today I filed the accounts payable info. It was the check stubs and bills that were already payed. I was looking at one particularly large bill and it was from the consulting company that our CEO is also CEO and managing partner of. Is it legal for him to be billing the our company through his other company. It seems like it is some sort of conflict of interest