Is working as an administrative employee in the General Customs Department in a Muslim Arab country forbidden by Sharia, and what is the expiation for that if it is proven to be forbidden?

The ruling on working in customs has been previously explained: if the work is permissible, then the salary is lawful; and if it is forbidden, then the salary is unlawful. The expiation for this is sincere repentance, leaving the job, and disposing of any amount exceeding one's needs.