The notice period/pay is part of service conditions that are governed by enactments applicable to the establishment e.g.
---Standing Orders (certified/model)...........if standing orders are not certified model standing orders shall apply. The notice period during probation period is NIL and 30 days after confirmation of service.
----(Name of the state) Shops and Commercial establishments Act:::: the notice period is 30days as per length of service
If the notice period of 15 days is not applicable to you as per the enactments the demand of HR shall not survive the test of law...
If no appointment letter stating notice period/pay was issued to you may mention that notice period/pay is not applicable to you as it was not part of the service conditions drafted by employer.
If it is applicable by all counts then demand FnF statement in original showing the adjustment of notice period/pay for verification and acceptance in which your pending wages/bonus/leave encashment etc has also been computed and also .....................request to supply by redg. post the acknowledgmentand aceptance of resignation,service certificate,relieving letter,Form16 as per correct FnF statement,Payment of FnF dues by bank DD only,PF number and a/c slips,salary slips of all months,ESIC card,NOC/NDC, within say 7 days by Redg. post.