Those in charge of this company should not mess about with this wealth, and they should put any surplus back into the company’s coffers. That is because this money was allocated to cover expenditures, so if the expenditures did not consume it all, it is not permissible to give it to someone who did not do any work. Rather they must return it, even if that means that they will not pay them extra in the following year or in other years. That is because they are entrusted with it, and the one who is entrusted with something must be honest with what was entrusted to him. If they need extra work to be done outside regular hours, then employees can do that work, and then be given as much as they deserve.
However, for the employees, if the managers of that department decide to do it in this way and give them money in this basis, they can accept it, based on what is mentioned in the hadith, that the Prophet (blessings and peace of Allah be upon him) said to ‘Umar (may Allah be pleased with him): “Whatever wealth comes to you when you are not hoping for it or asking for it, then take it, otherwise do not hope for it.” End quote.
Shaykh ‘Abdullah ibn Jibril (may Allah preserve him).
Fatawa ‘Ulama’ al-Balad al-Haram (p. 357).