Where the taxable person is unable to determine the value of goods or services or both or determine the rate of tax applicable thereto, he may request the proper officer in writing giving reasons for payment of tax on a provisional basis and the proper officer shall pass an order, within a period not later than ninety days from the date of receipt of such request, allowing payment of tax on provisional basis at such rate or on such value as may be specified by him.
(ICAI FAQ PUBLICATIONS 06-09-2017 PROVISIONAL ASSESSMENT: FAQ NO. 1)