How to get early release from project?

I have got an offer letter with 100% hike they want me to join within 30-60 days and my project got billable on 25 dec 2026. But this project management said no for early release and I don’t have any dependencies and project just started and I have also submitted my resignation. Can any one give any solution for this I have also asked for buy out but they are still saying no. Please give suggestions.