There may be included in the annual budget additional compensation for payment to the Prince George's County Department of Health and Department of Social Services employees to be known as State/County differential compensation. The amount of such compensation for each employee shall be determined by resolution of the County Council or set forth in the annual budget of the County. The total of such compensation for all employees shall not exceed budgetary limits. The amount of compensation for an employee shall not exceed the prevailing salary, as is reasonably determined by the County Council to exist at the time of such determination within the Washington Metropolitan area, for the job classification of the employee, less the amount of salary received from the State. Compensation may also be limited to employees hired before a given date or employees working in critical job categories.