The amount of a shift pay differential shall be determined by the Director of Classification and Compensation, with the approval of the Director of the Budget. In cases where the amount of the shift pay differential for a given class of work is not the same for all shifts other than the normal day shift, employees will be paid the amount of differential which has been authorized for work on the shift to which they are regularly assigned as defined in 143.1 (e) above. An employee re-assigned to work regularly on another shift for which a different shift pay differential has been authorized shall be paid the amount of the shift pay differential authorized for work on the shift to which he is so re-assigned.