Is this valid If they do not any proof..

Hi I am an employee in an IT company.. where I didn't sign any bond..I left job with all formalities.. they are asking me to pay amount to get relieving letter..when asked to ask to show the bond they are saying they will not show or send any copy..I have asked in written proof. please suggest how to prevent from this.. is this valid if I go to police give complaint as a blackmail.