Company unable to give reliving letter and treating abconding.

Sir's/ Madam's, I worked with a company for 2 years and received a good offer last November , I resigned with my company but manager was not accepting my 30 days notice period and asked me to serve 60 days notice period, I was shocked to here that I was entitled to serve 60 days notice period wherein the offer letter which I received from the company on email had 30 days notice period and when I joined the company they have made me signed a 60 days notice period which I was not at all aware, but then too I asked my manager if I can serve a 30 days notice and rest can be paid off via leaves and paying company back for which he said it will be depended on the transition of your work but will accept you resignation and the last working date will be said to you later depending on the transition process. I did all the transition and later after 23 days of notice period which I served without taking leaves which I had I mailed to my supervisor about the transition complete and my 30 days was almost near which he replied that you have to serve 60 days notice in which no wavier would be given to you. the offer I received was very good for my future and didn't want to loos the opportunity so I took a step and left the company on the 30th day of my notice period where my manager and HR told that I would be marked as absconding. now they have send me a F&F where my resignation date and date of leaving company is same and I am treated as absconding in spite of serving 30 days notice. now they are asking me money through F&F and I am ready to pay that too but they are saying I will not be getting a reliving / experience letter instead will get a NOC only. any suggestion will help me to take this up with the HR and supervisor.