Regarding Relieving Letter and Experience Letter

I am working with a company for 7 months. They confirm an employee after 6 months of probation period. They delayed my confirmation meeting and so, I am on the extended probation period. In my appointment letter, it is written that I have to serve 1 month notice period on probation and 3 months notice period after being confirmed. But, later they updated in their HR manual on the intranet that all the employees on probation as well as confirmed employees have to serve 90 days notice period. But, I was notified about this change in any way. I resigned and asked them to relieve me in 30 days. Now, they are not ready to relieve me in 30 days. They are saying that they won't leave me till they get a proper replacement for me. The updated HR manual says that if an employee is on an extended probation period, the company can terminate me in 7 days and I can leave the organization by serving 30 days notice period or salary thereof. I have another opportunity in my hand. I am ready to serve 30 days notice period. Now, if they won't give me a relieving letter and experience letter, what do I do?