89. Where under section 86 or 88 tax is payable on a particular day and the value of the consideration, or any part thereof, for the taxable supply is not ascertainable on that day,(1) tax calculated on the value of the consideration or part, as the case may be, that is ascertainable on that day is payable on that day; and
(2) tax calculated on the value of the consideration or part, as the case may be, that is not ascertainable on that day is payable on the day the value becomes ascertainable.