Is the money that the friend, who is a supervising engineer, took unlawful if the client withheld part of his dues due to disputes over raw material prices and the existence of defects in some works, knowing that the friend might have exaggerated the prices of some services?

It is permissible to work as a supervisor for finishing and furnishing apartments for a known fee, but it is not permissible for the fee to be a percentage of the final cost for the following reasons: ignorance of the wage at the time of contracting, and consuming wealth unlawfully because supervisory work does not differ with varying costs, and this may lead to lying and deception to increase the cost, or pushing the client to choose lower quality items.

Furthermore, the supervisor is obligated to return what was taken wrongfully to the client, which is the difference between the agreed-upon price and the actual cost of labor, because the supervisor is an agent for the client and is not allowed to profit without the client's knowledge. As for carpentry, the supervisor is not responsible for anything concerning it as long as he did not supervise its acceptance, and the client should demand the carpenter rectify the work.