The purpose of the notice period is to find a better replacement and ensure a smooth handover. In your case, a replacement is in place, the handover is completed, and you are also ready to buy out the 30-day notice period, which is fair on your part. I could not find any logic in holding you back.
Let's see what you can do now:
a. Just try to escalate this to the next level in HR/management of your present company and see if any positive outcomes occur. If you have already done this and not seen any positive results, try the following:
b. Please speak to the HR of the new organization where you are going to join and try to make them understand the situation.
c. Your current company can only withhold the experience and relieving orders if you leave without completing 90 days and nothing else. They cannot even withhold any statutory payments due to you. In fact, as per the appointment order, it is clear that you can buy out the notice period in case of inability to serve the notice.
d. Hence, please try to understand whether your new company will accept you without the experience and relieving order. If they are okay without these, you can proceed further to join your new company.
e. If you decide to leave as indicated in "c," send an email to your present company detailing the reasons for your improper departure as per their company policy. This may help in the future.
Though experience and relieving orders are important as you progress in your career, this is the only option you have now to move forward.
Regards,
Kannan M