Just skip the rehab step entirely and just release them - it's cheaper for the prison and the increased chance of re-offending means they'll get another lump sum next go around too!
That's not up to the prison. The only way for a convict to leave prison early is to convince the State that they are worth paroling.
Therefore, the prison would be incentivized to work with the prisoners to encourage good behavior and to show the parole board that a prisoner is worth paroling.
