GST ON ADVANCE BILLING

Charge full rate of GST as you have already issued Invoice. As per SECTION 13 of the CGST Act 2017, "TIME OF SUPPLY OF SERVICE", the Time of Supply of Service shall be Earlier of the following-

(1). Date of Invoice/Date of Service

(2). Date of Payment.

Since you have issued the invoice earlier, your time of Supply of Service will be the date of issuing Invoice and accordingly you have to file your Return and pay the applicable Taxes.

[ NOTE : W.e.f 15/11/2017, No Tax shall be collected/paid on Advance Received for SUPPLY OF GOODS...]

I think it is correct explanation, as Sec. 31 2 Tax Invoice -says bfore or aftet provision of service which means Tax Invoice can b issued bfore prov of Service. Thanx