Regarding notice from my previous employer

I was working in Krishi Vigyan Kendra under an NGO named Pragati trust. after joining at the Krishi vigyan Kendra , I got government job and I left the job immediately did not serve for notice period. In my appointment letter from the KVK it was written the if I did not give noticea before 1 month of leaving the job , they will hold my one month salary. and If i left the job before 2 years, then they have right to take 50000 from me. they sent me a notice from the KVK signed by the KVK Head , not signed by the trustee of NGO. Please tell me what should I do.