Section 24. The authority is hereby authorized and directed to reimburse the commonwealth for the amount of retirement costs incurred by the commonwealth on behalf of employees of the department of public safety for the time such employees are assigned by the commissioner of said department to duty with the authority. Such amount shall be the retirement cost portion of the cost of fringe benefits as determined by the commissioner of administration pursuant to section six B of chapter twenty-nine. Such amount shall be reimbursed annually to the commonwealth for fiscal years beginning after June thirtieth, nineteen hundred and eighty-seven.