What is the ruling on giving a tip to someone who performed a service and receives a fixed salary for it?

It is not permissible to give a gratuity or a gift to an employee who receives a salary, based on the Prophet's (peace be upon him) saying: "What is wrong with a worker whom I send, and he says: 'This is for you, and this was gifted to me'? Why did he not sit in his father's house or his mother's house to see if anything would be gifted to him or not?" And also based on his (peace be upon him) saying: "Whomever we appoint to a job and provide him with a salary, whatever he takes after that is ill-gotten gain (ghulul)." Therefore, gifts to employees are unlawful, and the ruling concerning them is that they must be returned to the giver. If that is not possible, then they should be given to the public treasury (Bayt al-Mal). This applies to a government employee, for whom it is not permissible to accept any money or gift due to his position, whether it is in the name of a gift, charity (sadaqah), or zakat, because this opens the door to bribery, which is one of the major sins.