Unfair Project Hold: Seeking Support for Timely Release and Career Growth

My manager approved my release from the project, but he is asking me to continue working for an additional 4–5 months before the release. He hasn't mentioned an exact release date in the email. I told him that if I get released, I can join another project and work towards improving my band, but he didn’t agree. Can you help me negotiate this and get an early release within one month?