Is it permissible for a purchasing agent to keep the excess money that the shop owner returns to him, especially since this money cannot be returned to the company and he spends it on work expenses?

A gift taken by an employee from clients is a form of bribery and an incentive for favoritism and disloyalty in work. It was narrated from the Prophet, peace and blessings be upon him: "Gifts of workers are ghulul (betrayal/embezzlement)," and ghulul is treachery. It is not permissible for an employee to accept gifts from clients, as this is a pretext for injustice and fulfilling needs unlawfully. An employee who deals with the public should not accept gifts from those he interacts with, because he will show them favoritism. It is also not permissible to give an employee a sum of money as a tip if he has a fixed salary, as it corrupts him towards others and sets a bad precedent. Therefore, it is not permissible to take this gift from the client, and one must refrain from taking it in the future, and seek forgiveness and repentance for what has passed.