What is the ruling of Islamic law regarding a profit margin ranging between 10-40% of the value of the commodity, and must it be returned to the owners of the money given the impossibility of knowing its exact value?

If you are appointed as an agent to purchase an item, it is not permissible for you to quote a higher price and take the difference, because an agent is entrusted, and you must return the money you took. However, if they ask you for specific goods, and you purchase them and then sell them to them at a higher price, there is no harm in that.