Company trying not to give me notice period & buy out option

I have got an offer from another Company and I have resigned from my Current Company agreeing to serve one month of notice period and will buy notice of one month by paying the required amount of one month. As per my company policy, staff needs to serve 2 months of the notice period or staff can buy out 1 month of notice. There is no where written in Company policy that it's mandated to serve 2 months of the notice period. My manager has no issues to let me go by serving 1 month of the notice period. Now HR is saying we will see if we can allow you to give buyout option or not. HR is not clearing that I will get release within one month or I have to extend for another one month. I just want to be prepared beforehand if HR creates any issues later. I need to have my release letter by serving one month notice only and paying remaining one month salary for buying notice. Could you please advise me on this. Any help is highly appreciated. Thank you.