It is not permissible to give tips or gifts to workers who get a salary for their work. The evidence for that is what was narrated by al-Bukhaari (6578) and Muslim (1832) from Abu Humayd al-Saa’idi who said:

The Messenger of Allaah (peace and blessings of Allaah be upon him) appointed a man from al-Asad who was called Ibn al-Lutbiyyah. When he came he said: This is for you, and this was given to me. The Messenger of Allaah (peace and blessings of Allaah be upon him) stood up on the minbar and praised and glorified Allaah, and said: “What is the matter with an agent whom I send, and he says, ‘This is for you and this was given to me’? Why doesn’t he sit in the house of his father or the house of his mother and see if he is given anything or not. By the One in Whose hand is the soul of Muhammad, no one of you gets anything from it (unlawfully), but he will bring it on the Day of Resurrection, carrying it on his shoulders, even if it is a groaning camel, a lowing cow or a bleating sheep.” Then he raised his arms until we saw the whiteness of his armpits, then he said: “O Allaah, have I conveyed (the message)?” two times. 

Al-Nawawi (may Allaah have mercy on him) said: This hadeeth indicates that giving gifts to workers is haraam. Hence in the hadeeth he mentioned his punishment and the fact that he will carry what was given to him on the Day of Resurrection. The Prophet (peace and blessings of Allaah be upon him) stated in the same hadeeth the reason why it is haraam to give him gifts and that is because he is appointed to do this job, and this is unlike giving gifts to someone other than a worker, which is mustahabb. The ruling on that which a worker takes in the name of a gift is that it must be returned to the giver, and if that is not possible it must be given to the bayt al-maal. 

Sharh Muslim (6/462). 

It was narrated from ‘Abd-Allaah ibn Buraydah from his father that the Prophet (peace and blessings of Allaah be upon him) said: “Whoever we appoint to do a task and we give him his provision, whatever he takes after that is ghulool (stolen booty).” What this hadeeth means is: whoever we appoint for a task and give him money for that, it is not permissible for him to take anything after that. If he does take anything, it is ghulool, which refers to wealth stolen from the war booty or the bayt al-maal of the Muslims. 

Narrated by Abu Dawood (2943) and classed as saheeh by al-Albaani in Saheeh Abi Dawood. 

These two hadeeths show that it is not permissible for one who is employed and receives a salary from his workplace to accept money or gifts from anyone because of his work. If he does that, it is ghulool. 

Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) was asked: We are government employees and we get tips and zakaah from some businessmen during Ramadaan, and we cannot distinguish between the tips and the zakaah because we do not know what is what. 

Our question is: If we take this money although we do not need it, and spend it on widows, orphans and the poor, what is the ruling on that? If we spend any of it on our families and consume it, what is the ruling? 

He replied: 

Giving gifts to workers is a kind of ghulool, i.e., if a person has a government job and someone who has some business to do with him gives him a gift, this is ghulool, and it is not permissible for him to take any of it, even if it is given willingly. 

For example: let us assume that you have dealings with some department, and you give a gift to the head of this department, or to his employees. It is haraam for him to accept it, because the Prophet (peace and blessings of Allaah be upon him) sent ‘Abd-Allaah ibn al-Lutbiyyah to collect the zakaah and when he came back he said, “This was given to me as a gift and this is for you.” The Prophet (peace and blessings of Allaah be upon him) stood up and addressed the people and said: “What is the matter with a man among you whom we appoint to do a task, then he comes and says, This is for you and this was given to me. Why doesn’t he sit in the house of his father and mother and see whether he is given anything or not? 

It is not permissible for any employees in any government department to accept a gift in any dealings that have to do with this department. If we open this door and say that it is permissible for an employees to accept this gift, then we will have opened the door to bribery, and bribery is a very serious matter and a major sin. If employees are offered gifts that have to do with their work, they should refuse these gifts, and it is not permissible for them to accept them, whether they come in the name of gifts or charity or zakaah, especially if they are well off, because zakaah is not permissible for them, as is well known. 

Fataawa al-Shaykh al-‘Uthaymeen (18/ question no. 270) 

And Allaah knows best.