Experience letter not given by company how can I get it

Sir, I worked in a private company and left without serving the notice period of three months. In my appointment letter it is clearly written that I am supposed to either serve a notice period of three months or I can pay the salary of three months. I proposed paying three months salary they didn't agree and my resignation remained unacceptable and I left. Then later I got letters from company saying I was absconding though I had written applied for resignation and informed them about leaving. Now they have terminated me and not ready to give experience certificate. What can be done?