In the agreement the board of supervisors shall agree not to reduce the salary of the commissioner to less than the base salary. The director may agree to pay to the county a sum not to exceed six thousand six hundred dollars ($6,600) per year to be used for the salary of the commissioner, or to compensate the county for services which are performed for the department. The amount which is paid to a county shall not, however, exceed two-thirds of the amount of the salary which is paid to its commissioner.