Is the extra amount (300 EGP) taken by the teacher, who swore he only took 100 EGP, permissible or forbidden?

The person who requested the assembly of the device and the teacher was aware of all this and was hired for it, then the teacher is an agent and a trustworthy employee. His duty is to work for the benefit of the principal. The extra amount he took contradicts this. Therefore, he must return the money to its owner, along with repentance and seeking forgiveness. It is not a condition to inform the owner about returning the money. He must also repent from the false oath by which he wrongfully acquired a Muslim's property, as it is one of the grave sins.