Should the salary be returned to the school after the end of a period of employment tainted by injustice, or is giving it away in charity sufficient?

If showing off (riya') and injustice (zulm) do not affect one's salary, then one must seek forgiveness from those who were wronged. However, dereliction of duty in work does affect one's salary, and it is obligatory to repent and return the corresponding amount to the school, unless one has been absolved of it. Giving it away as charity is not sufficient as long as returning it is possible.