due date of payment for supply of service is 28 july (ascertainable through contract )recipient made payment on 21 july when should the tax invoice be issued
Invoice has to be issued within 30 days from the completion of service. You can issue prior to that. On your case the 28& 21 both are in same month so there is no issue. Its up to you. If month is different then there would be issue.
As per Section 13 of CGST Act : the time of supply in your case will 21july , GST liability in case of supply of service is arises on the date of payment reced or Invoice issued , whichever is earlier .
Invoice can be issued on date when the Services has been provided.