My Employer is threatening for legal action after submitting resignation

I got a better opportunity in another company and my new employer wanted me to join ASAP so gave a formal resignation to my current employer. As per my joining letter (2.5 years old), there is 45 days salary in lieu of notice is mentioned and as per my latest increment letter (6-month-old), the notice period is 2 months. So my current employer is forcing me to serve 2 months notice period but I gave them 15 days from my side and clearly mentioned I will leave my position after 15 days as per the resignation submission date and offered them to buy out the rest of the notice period. But I had extended it to 24 days to complete pending projects and left after delivering all the assets of the company. I had submitted my resignation on 17th Jan and left my job on 11th Feb after serving 24 days notice period they already held my 17 days salary for January. Now they are denying to pay my salary and providing a reliving and experience letter if I will not serve 2 months notice period and threatening me to take legal action. 1. Can you please help me to understand, am I able to claim my salary and experience letter? from my old employer. 2. I had 17 days salary+24 days notice+13 casual leaves that are making 54 days of salary. can i claim this? 3. I Already offered them at the initial stage to buy out the notice period as my new employer was ready to pay for it but they didn't provide an answer on that so now can they claim any money from me? Please help.