Employer is forcing me to serve three months notice period?

Hi.. My name is sagar chavan working in pune with just engineering pvt ltd from 22th september 2014 my last day of probation was 22th march 2015. two day before my probation period on 20th march 2015 I have resigned with proper resignation email to my manager to relieve me next 10 days. as there is notice period is not mentioned anywhere clearly in my appointment letter. After a meeting with my manager and HR ,they are forcing me to serve notice period of thee month or ask your future employer to buy back notice period. i clearly told them i can still serve you till 20th april 2015 which is one month notice period though it is not mentioned in appointment letter. They even refused this option and forcing me to serve 3 month notice period.i have given them also assurance, if 3 month notice period is mentioned clearly anywhere in my appointment letter i am ready to serve if that condition is mentioned in appointment letter. As such all conditions has already mentioned with all appointment letters with my previous experiences. Now they are saying there is point number 8 in my appointment letter which states "you will be governed by service rules and regulations of company in force and as ammended from time to time." As per them,This point menas you have serve three month notice period. they never shown what are the conditions coming under service rules and regulations while tendring offer letter as well as appointment letter. This is kind of mental harassment now from my employer. they said if you are not ready to serve three months notice then we will not releasing my document and my holding up my salay Which is 85000 INR For one month. Please let me know,what should i do in this case. Is it possible that i can take any legal action against them. Thank you, Sagar Chavan.