Suppose the GST rate on a service rises from 5% to 18% with effect from 1 September. A service worth ₹10,00,000 (excluding tax) is supplied and invoiced on 4 September, but full payment was received…

Section 14(b)(iii) of the CGST Act, 2017 covers a supply made after a rate change where the invoice is issued after the change but payment was received before it: the time of supply is the date of the invoice, so the new rate applies and tax is 18% of ₹10,00,000, or ₹1,80,000. The earlier payment date would govern only where the supply and invoice both came before the change.