Point of taxation: invoice date governs tax point when recipient liability changes but payment remains unpaid. Where there is change in the liability or extent of liability of a person required to pay tax as recipient of service notified under sub section (2) of section 68, in case service has been provided and the invoice issued before the date of such change, but payment has not been made as on such date, the point of taxation shall be the date of issuance of the invoice.