Is it permissible for me to benefit from a discount given to me by a supermarket employee without the owner's knowledge, especially since the supermarket offers discounts to other customers, but the owner does not grant me any discount if I buy directly from him? And what must I do regarding what I have taken if it involves a sin?

There is no objection to accepting the discount offered by the employee, as long as he is authorized to do so by the shop owner. This is evident if he grants the discount to all customers. However, if it becomes clear that the employee is giving the discount without authorization from the shop owner, then it is not permissible to accept it except with the shop owner's permission; because an agent does not have the authority to act beyond what his principal authorizes.