Regarding relieving from my current organisation

I have joined a small-mid scale company on 10th May 2016 on a 6 months contract. Contractual agreement clearly states that contract is terminable by giving 30 days notice. Now I have a very good opportunity to join another organisation. I had discussed this multiple times with my manager who's also co-director of this organisation but without any conclusion. I have finally written my resignation via email on 17th of August. And I have committed my joining to the new employer on 19th of September considering my notice period. But still my current employer is asking me to extend my stay for another one month, which is not acceptable by my new employer, understandably so. I really need to join the new organisation for my career growth. Now I need to submit my relieving letter to my new employer or at least an acceptance similar to that sort, which I think my current organisation will deny. I heard many stories that the company has a reputation for misusing their employees this way and leaving them without any full and final settlement. I did not know this before and also that the co-director has good connect with police and law-firms. I do not want to go by legal way unless really need to, since this takes time and its a tiring process. Please suggest any alternate way of resolving this situation.