(a) Notwithstanding any provision of this code to the contrary, in lieu of overtime compensation, employees of higher education organizations may receive compensatory time off at a rate not less than one and one-half hours for each hour of employment. Employees may receive compensatory time only under the following conditions:
(1) The time is awarded pursuant to a written agreement between the employer and the employee arrived at before the work is performed. A written agreement may be modified at the request of the employer or employee, but under no circumstances may changes in the agreement deny an employee compensatory time already acquired;