Fig. freedom to exercise complete control over something. (*Typically: get ~; have ~; give someone ~.) I didn't get a free hand with the last project.John was in charge then, but he didn't get a free hand either.
Also, free rein. Freedom to do or decide as one sees fit. For example, The teacher gave her assistant a free hand with the class, or They gave me free rein to reorganize the department. The first expression dates from the late 1800s, the second from the mid-1900s.