
Why managers prefer not to release employee
Why managers prefer not to release employee from project rather just keeping him.... When even employee don't want to work in specific project.

Resource hiring and allocation is indeed a costly process though it's not visible to the associates. What you can do is - clear your expectations, propose to train someone taking out some extra time, assure that post release you would be supporting to the best of your capability until your backup is upto speed. Post these assurances, ask for a tentative release date. This is the right way to go about it.
But if your management is toxic, doesn't want to listen to your expectations at all in assuring a proper work environment for you - then simply make your unwillingness to work felt in the project 😄

What if I give them internal resource, will that be helpful enough

So how to crak it then?