Get in touch with Unions/Labor Inspector (of appropriate govt in your case i.e. state/central )/ Inspector appointed under Shops and Estbs Act (that might be Labor Inspector appointed by state Govt.)..
Grievance Redressal Committee if IT is notified in your state..
And a very able Local senior counsel of unshakable repute and integrity specializing in Labor/Service matters and well versed with LOCAL applicable rules/laws and having successful track record…. and worth his/her salt…………………..
Your counsel(s) may opine to ask for salary for period worked, correct FnF statement, service certificate/relieving letter (without any adverse comments) and to withdraw said legal notice…