Shitty office politics

I am works in a production support project. I asked release from project as my aspiration was to work in a data science project. I had started completing courses for the same. When I asked for a release they said a straight no as my manager was going to to on a maternity leave. They also expected me to fill her position for a while for which I completely disagrees as there is a big pay difference between us. Then I put a resignation letter. After almost 2.5 months I contacted the HR stating I am willing to revoke my resignation if I am released from the project. But now they are saying they dont have my backup. Even 2.5 months was not enough to find a backup. And they are saying if I had left the company they can work without filling my position, but if I revoke they want me in the project for the next 3-6 months. What kind of injustice is this????