He was sent overseas to study and they gave him a complete salary by mistake
Question

                I have been sent overseas to study and I am also an employee. The system is supposed to give me half of the salary I get in Saudi, but until now they have been giving me the salary in full. Please note that I have done everything that is required to tell the company so that they will deduct half of the salary. What should I do? Please note that they may discover that later on, then they will deduct it from future salaries, as happened to my colleagues. Is this money halaal or haraam? Should I tell them about it? Please note that I am worried that the person who made this mistake will be punished.
Praise be to Allah.
      If the system dictates that you should have half of the 
      salary whilst you are overseas, it is not permissible to you to take 
      anything more than that. What you have to do is tell those who are in 
      charge, regardless of whether the person who made the mistake will be 
      published or not. 

    Shaykh Ibn Baaz (may Allah have mercy on him) was asked: I am 
    an employee working in a government department and sometimes they pay us 
    overtime without us having worked overtime and without us having attended 
    for work, and they regard that as a bonus for employees from time to time. 
    Please note that the head of the department is aware of that and approves of 
    it. Please advise us, may Allah reward you with good: is it permissible to 
    take this money? If it is not permissible, what should I do about money that 
    I have taken in the past, which I have disposed of? May Allah reward you 
    with good. 

    He replied: If the situation is as you have described, that 
    is wrong and is not permissible; rather it is a betrayal. You have to give 
    back whatever you have taken in this manner to the state funds. If that is 
    not possible, that you have to give it in charity to poor Muslims or to 
    charitable projects; you also have to repent to Allah, may He be glorified, 
    and resolve sincerely not to do that again, because it is not permissible 
    for a Muslim to take anything from the bayt al-maal (treasury) of the 
    Muslims except in legitimate ways that the state knows of and approves of. 
    And Allah is the source of strength. 

    End quote from Fataawa al-Shaykh Ibn Baaz, 19/344 

    And Allah knows best.