Sir,
According to Section 31 (5) of CGST Act, 2017-
Subject to the provisions of clause (d) of sub-section (3), in case of continuous supply of services,––
(a) where the due date of payment is ascertainable from the contract, the invoice shall be issued on or before the due date of payment;
(b) where the due date of payment is not ascertainable from the contract, the invoice shall be issued before or at the time when the supplier of service receives the payment;
(c) where the payment is linked to the completion of an event, the invoice shall be issued on or before the date of completion of that event.
Clause (d) of sub-section (3) of Section 31 of CGST Act, 2017 stipulates that-
a registered person shall, on receipt of advance payment with respect to any supply of goods or services or both, issue a receipt voucher or any other document, containing such particulars as may be prescribed, evidencing receipt of such payment.
Therefore according to the above provisions you have to issue the invoice in your case.