Resignation notice not being accepted

Hi, I was recently promoted to an Assistant Manager in a corporate company, I got a better opportunity now and have sent a resignation email stating that I am serving a 30 days notice as per my initial offer letter and grade which I signed, my HR is now informing me the notice period for my grade is 90 days, I have disputed this saying this is nowhere mentioned in my promotion letter or my compensation revision letter nor any intimation was given by the organization, the letter just mentions all other terms and conditions are applicable as per your grade, I have asked for these so called terms and conditions, but have not been provided with this, over all the seperation policy document available has no mention of the notice period for any grade of employees. The only intimation the employee gets is when he goes to submit an online resignation which shows last working day as 90 days later, this seems like malpractice to me, what the most the company can do if i don't serve full notice, I have even offered and willing to buy out the remainder of the 90 days period however no one is able to provide me with a policy document showing the notice period.The document mentions that notice period is applicable as per the offer letter or will be intimated by the company from time to time, none of which were done.