Is success in a job interview considered lawful (halal) or unlawful (haram) if an intermediary (wasta) intervened for the applicant's benefit, and what should one do if it is unlawful?

There is no objection to taking on the job if you are qualified for it and pass the test without cheating, unless there is someone more deserving than you. The intercession or mediation of the manager is permissible and religiously sought in matters of goodness, for the Prophet, peace and blessings be upon him, said: "Intercede, and you will be rewarded, and Allah will decree through the tongue of His Prophet what He wills."