Every employee granted a leave of absence pursuant to this article may be required to perform such services during the leave as the governing board of the district and the employee may agree upon in writing. The employee shall receive such compensation during the period of the leave as the governing board and the employee may agree upon in writing, which compensation shall be not less than the difference between the salary of the employee on leave and the salary of a substitute employee in the position which the employee held prior to the granting of the leave. However, in lieu of such difference, the board may pay one-half of the salary of the employee on leave or any additional amount up to and including the full salary of the employee on leave.