Is it permissible to give an incentive (a "sweetener") to an employee who requested it after products had been supplied to his company, knowing that his role is limited to bringing in the supplier and accrediting them, and that the questioner refused the request but the employee claimed it was permissible?

It is not permissible to give an employee anything because of their work, nor is it permissible for the employee to request it. This falls under bribery and gifts to workers, which are considered embezzlement, in accordance with the Prophet’s (peace and blessings be upon him) saying: "Gifts to workers are embezzlement," and "May Allah curse the one who gives bribes and the one who takes them." This action is considered an attempt to gain favoritism from the employee and influence them. Islamic law has prohibited gifts to employees to prevent bribery, as did Umar ibn al-Khattab (may Allah be pleased with him) when he considered gifts to workers as embezzlement.