Is taking 5% of the payment commission allocated to a third party considered unlawful (haram), given that the employer is aware of the change in the nature of the amount from a bank tax to a personal commission, and without requesting the percentage from the financier?

If your employer tasks you with providing a payment method, it is not permissible to take a fee from someone you know without your employer's permission. However, if you yourself direct one of your acquaintances to your employer outside of working hours, there is no sin upon you in taking a commission from that person, provided there is no lying or deception towards the employer.