At campuses where balanced calendar operation has been approved by the board of trustees, any academic year employee, subject to the approval of the chancellor of State University, may be required or permitted to render service in excess of such professional obligation. Such approval shall be granted only in unusual circumstances where, in a two-year period, the needs of the campus require professional service beyond the professional obligation normally established for such period. Any academic year employee who performs such additional approved service shall receive additional compensation at not to exceed the following rates: