What is the ruling on accepting a sum of money as a gift from a seller after returning an item to him due to its non-compliance with Sharia?

What an employee receives from a seller as a gift is considered a forbidden bribe, because he takes it for his obligatory work for which he receives a salary. In the Hadith: "Gifts of employees are ghulul (ill-gotten gains)." Therefore, anything added to his designated salary is unlawful. If the employee directed the seller to raise the price of the item so that he could take the difference, this is clear ghulul, and the way to be absolved of its consequence is to return the amount to the employer.