Unable to serve notice period and pay the amount.

Greetings , I hope you find this in your good ethos. I am , working in MNC would like to seek your suggestion on my case. I joined a private company in Chandigarh and worked for 9 months. On 16, Jan 2017 I rendered my resignation and asked them to relieve me as soon as possible as the next company was giving me a great opportunity and wanted me to join immediately. Additionally the work environment was deteriorating day by day , so I took an opportunity and left the former after giving a proper handover and serving 2 days of notice period. The Manager had accepted my resignation which mentioned that since I am not serving my notice period , they will not be able to provide any kind of full and final payment and relieving letters. Additionally there was no retention or the negotiation meetings done in that period of time. I still had attended an additional day in that office hoping to have a meeting for the same. However there was no such meeting. Now after 6 months , I have received a letter from the company saying that I am liable to pay a sum of 37770/ INR to the company since i was not able to serve the notice period.Else they will take a legal action. I would like to raise the following point , on which I seek your guidance. 1. Since the resignation was accepted , there and then , which implies that they had accepted that I ll leaving without notice period. So why and how I am liable to pay the amount. 2. I have left my 16 days salary along with 15 days leaves, so on what grounds I am liable to pay them the amount. 3.In-case I was liable to pay the amount, then why the letter was not issues at that very moment. I am hoping that I have provided you with all the relevant information that would help you to analyse the same Looking forward for your reply. Thanking you.