(1) The Authority may estimate the charge payable in respect of the exercise or performance of any function, power, or duty and require that estimated charge or part of that estimated charge to be paid in full before the Authority exercises or performs the function, power, or duty to which that charge relates.
(2) Where the actual and reasonable costs of exercising or performing any function, power, or duty,—
(a) exceed the amount paid in advance, the difference between the amount paid and the actual and reasonable costs shall be a debt and the provisions of section 21(5) shall apply:
(b) are less than the amount paid in advance, the Authority shall refund the difference between the amount paid and the actual and reasonable costs.