Important: GST on advances for goods (not services) was exempted by Notification 66/2017 for businesses not required to issue e-invoices. If you are not in the e-invoicing ambit, you can issue the tax invoice only at the time of supply of goods — even if advance was received earlier. For services, GST on advance applies with no exemption. Most professional service firms (consultants, IT companies, architects) receive advance retainers and must issue receipt vouchers and pay GST on those advances.
Example
A Mumbai architect receives ₹2,00,000 advance on a ₹10,00,000 project. Receipt Voucher issued: advance ₹2,00,000, GST 18% = ₹36,000 (CGST ₹18,000 + SGST ₹18,000). When project deliverables are completed, Tax Invoice issued: ₹10,00,000 total, GST ₹1,80,000. Less advance adjusted: ₹2,00,000 + ₹36,000. Balance due: ₹8,00,000 + ₹1,44,000 GST.