Where an employee becomes entitled to a period of long service leave under this Act, he may, by agreement with his employer, elect to accept payment in lieu of the period of long service leave to which he is so entitled and if he so elects the employer may pay him accordingly a sum equal to the amount of his ordinary pay in respect of a period commencing on the date specified in the election, or, if no date is so specified, the date of the election, and of the length of that period of long service leave.