What is the ruling on an architect receiving a fee for signing off on supervision documents for structures he did not oversee, knowing that government employees are aware of this?

Your signing off on supervision documents for sites you did not supervise is a false statement and a testimony for which you will be held accountable on the Day of Judgment. The money you receive for it is illicit gain. You must repent to Allah, inform the relevant authorities of the truth regarding your lack of supervision, and dispose of the money you obtained in this manner by spending it on the poor and needy. Indeed, retracting false testimony is an act of repentance for the false statement that was made.