Is the salary I receive from my job considered permissible (halal) or forbidden (haram), knowing that I obtained it through a second appointment in a government job competition after the first candidate resigned, and I entered the competition through mediation? And am I an oppressor because of these circumstances?

Since the matter has been referred to the judiciary, it is authorized to clarify who is entitled to continue in the position. There is no blame upon you for the salary you receive until the ruling is issued, as long as you are competent for this job and perform it as required. And seek forgiveness from Allah for any transgressions that occurred in the terms of appointment or on the day of starting work, and do not repeat similar actions.