In terms of Section 13 of the CGST Act, 2017, the criteria to decide time of supply of services includes the date on which payment is received by the supplier. Accordingly, the service provider should remit the applicable taxes on such advances in the month in which the money is received in advance even otherwise the services are not supplied / provided.
Subsequently, when the invoice is issued with respect to the advance payments received earlier, the same shall be declared in the returns pertaining to the month in which the invoice is issued, by giving reference of the ‘Transaction ID’ generated at the time of remitting taxes on the advance payments (in the earlier tax periods).
(ICAI FAQ PUBLICATIONS 06-09-2017 TIME OF SUPPLY FOR SERVICES: FAQ NO. 17)