Why are you continuing with the job?
Understand the law. You telling you have resigned. Then how you can be on the roll of employees? Is it not confusing?
Relieving is a complete different issue. Company may not relieve you till dues if any, are not cleared by you, which shall remain open to be settled through process of law.
Once your resignation letter is received by your employer, employment relationship shall end with effect from the date of resignation mentioned therein.
Now the clause that buy out option shall be at the discretion of the management, is totally illegal and shall not have any legal force. It is void because management can not force you to serve for a specific period of time.
What management can do is to demand you to pay up for the unserved notice period, if any.
Advice:
You can easily join new job pending your relieving issues.
Inform your previous employer that you have joined new job and you shall on demand pay up for the unserved notice period, if any.
Do not make the issue complicated by your own. Law is very clear on this issue.
Nobody even govt of India can not force you to do job for a specific period of time.