Manager focing to extend notice period at the last moment

I am working with a reputed organisation as a permanent employee. My current notice period is 3 months. I got a good offer from some other organisation but they were not ready to wait for 3 months. I discussed this with my managers/HR and they confirmed over email (with HR in CC) that they would relieve me in 2 months (I have to pay for 1 month). As per this confirmation, 2nd party was ready to wait for 2 months and I accepted that offer. Now when my notice is about to get over, my manager is threatening me to extend the notice period. If it happens, I would end up having no job as 2nd party is not ready to wait for 1 more month. Please help.