Is it permissible to take 5 Egyptian pounds in exchange for repairing something, knowing that the owner of the item is unaware of this, and that the original agreed-upon amount was 20 Egyptian pounds, while the repair was actually carried out for 15 Egyptian pounds?

If there is an agreement to repair something for a known wage, it is permissible to hand it over to another to perform the repair and take the difference between the two wages, provided that you are not an agent for the owner of the item, or that the owner of the item did not stipulate that you perform the repair yourself.