Invoice and payment voucher when GST payable under reverse charge if supplier not registered :
A registered person who is liable to pay tax under section 9(3) or 9(4) of CGST Act [reverse charge] shall issue an invoice in respect of goods or services received by him on the date of receipt of goods or services from the supplier who is not registered under the Act – section 31(3)(f) of CGST Act.
Such invoice is required only when supplier is not registered under GST, as he is required to pay tax when supplier is not registered under GST.
When payment is made under reverse charge, payment voucher is required to be issued – section 31(3)(g) of CGST Act.