Company hold my Relieving letters and fnf amount.

Hello sir, My name is Raju, from anand Gujarat. I have Resign my job after working 3 years. I have serve full 1month of Notice Period also. when joining the company I had sign NDA papers. Now after my resignation letters In last week of my notice Period HR ask me to sign One Declaration letters which means I will follow all Clause, terms & Conditions as per my NDA papers but company changed Stamp Notary papers (issue new notary papers) & do notary registration before 1 Week of my resignation date. so I have refused to do signature on this declaration papers. due to this company hold my Relieving letters & fnf settlement amount. so can you guide me How can I take my Letters & fnf amount from Company? can any Company ask Employee to sign declaration letters after resignation? Now Company after 1 week send me mail & ask me to sign in declaration letters, if I am fail to do this then company will take legal action against me. can you please guide me for how to handle this situation.