Notice period buyout at basic salary or gross salary

I have resigned from a company stating that I will serve one month of notice period as I am still on a probation. But my appointment letter mentions a notice period of 3 months (does not specify probation or confirmation). HR is asking to serve the notice period of 3 months. I accepted the same (as it mentioned in my appointment letter) with buying out of 2 months’ notice shortfall (as I am already serving one month) with basic salary. Now company has denied my request stating that if I have to buy the notice period I need to pay gross salary, which is nowhere mention in my appointment letter. I also demanded my accrued leaves of 22 days be settled against my notice shortfall of 2 months. Company denied the same stating they do not provide leaves in probation period, whereas on my appointment letter it nowhere mentioned that I won’t get any leaves during probation instead it is mentioned that I am entitled to get 24 leaves in a year at pro rata basis (2 leaves/month). They have also stopped/held my salary after my resignation, which I need to survive for daily chores. Basically I am requesting them to follow what is mentioned on my appointment letter but they are playing rogue. They are taking undue advantage of my situation as I am requesting the relieving letter. How can I proceed further & what is the quickest action to get a relieving letter without paying the gross salary or in other terms what are the legal obligations I they are not complying to the terms & conditions of the appointment letter.