Is it permissible for the company to keep the price difference if it was able to purchase materials – such as ceramic – at a lower price than what was agreed upon with the client in the contract, knowing that the contract stipulates detailed clauses and a total cost approved by the client?

It is not permissible to work as a supervisor for apartment finishing in exchange for a percentage of the final cost. Rather, the wage must be a fixed and known amount from the outset. If the interior designer brings materials for the client as his agent, he is not permitted to profit from them without the client's knowledge, because an agent does not profit from what he has been entrusted with. However, if it is agreed that the engineer will undertake the complete finishing —labor and materials— for a specific sum, this is a 'contract of istisna' (manufacturing/construction agreement) in which he is not questioned about the price of the materials or his profit from them. In this case, it is stipulated that the agreed-upon characteristics and specifications must be precisely defined to prevent disputes.