The ruling on studying networking for one who will use it for haraam purposes
Question

                I am a network engineer I wants to do Masters thats why I went to UK but after going there I realized that the master course is very costly and people will work there and then pay money for their living cost and university fee.But the jobs which you can find is working in a bars and other jobs are like working in the shops even there you have to sell the Alcohols sometime or distributing the newspaper in which there will be nasty adds and there will be add of alcohol. I find a job after sitting in the home for 3 months. My job is related to teaching the Networking to non-Muslims my question is that they might learn from me something and tomorrow if they will go in the bars and do networking for them or they might go to banks and work there. If they work there after learning from me Am I get sins for that? But the thing is that if I will work in Muslim county or teach there as well than some of the Muslims they will go and work in the banks will i get sins for that one as well??

Another confusion which my friend have he says that if you are not working in the bank but if you work for the company who have contracts with different companies and they might send you in the bank to fix something like fixing their computer or do networking for them in the bank. Is this harram??

kindly shiekh give me answer according to quran and hadeeth .
Praise be to Allah.
      There is nothing wrong with teaching networking, even to 
      non-Muslims, because it has numerous uses and is not usually used for 
      haraam purposes, unless you know -- or think it most likely -- that the 
      student is only learning it in order to use it for haraam purposes, or 
      that this is usually the case for those who study that in that country. In 
      that case, it is not permissible to teach it, because that is spreading 
      corruption and helping in sin and transgression. 

    The basic principle to be followed in this matter is that it 
    is not permissible to be hired to do something haraam or to do something 
    that may lead to haraam, because when Islam forbids something it blocks the 
    means that lead to it. 

    Allah says (interpretation of the meaning):

    “Help you one another in Al‑Birr and At‑Taqwa (virtue, 
    righteousness and piety); but do not help one another in sin and 
    transgression. And fear Allaah. Verily, Allaah is Severe in punishment”

    [al-Maa’idah 5:2]

    Shaykh ‘Abd al-Rahmaan al-Sa‘di (may Allah have mercy on him) 
    said: With regard to every sin and wrong action, a person must refrain from 
    it himself, then he must help others to refrain from it.

    End quote from Tafseer al-Sa‘di, p. 218. 

    Imam al-Shawkaani (may Allah have mercy on him) said:

    Everything that a benefit depends on is permitted by 
    sharee‘ah: so selling it is permissible. And everything that has no benefit 
    at all, or that benefit is not permissible, then selling it is not 
    permissible, because the means that leads to haraam is also haraam. But it 
    is essential that the benefit in that thing should not be haraam in all 
    cases. 

    But if it is something of which the benefit may be halaal in 
    one case and haraam in another case, or it is something that may be used in 
    a haraam way or in a halaal way, if the seller knows that that purchaser 
    will only use it for haraam purposes, it is not permissible to sell it to 
    him; but if he knows that he will use it for halaal purposes, it is 
    permissible to sell it to him. If the matter is unclear and it is possible 
    to use it in both halaal and haraam ways, then it is permissible to sell it, 
    because there is no impediment to selling it, and uncertainty without 
    knowing what is more likely to be the case is not to be paid attention to. 

    Then he said, after mentioning the evidence for what he 
    stated: 

    To sum up, if it seems most likely that it will be used in 
    ways that are not allowed, which is the haraam benefit, it is not 
    permissible to sell it. The fact that it is most likely to be used in a 
    haraam way should make the seller think that the purchaser is only buying 
    that item for that haraam usage. But if that is not the case, then the 
    matter is as stated above. 

    An example of that is selling grapes and dates to one who it 
    is thought most likely will use them to make khamr, or selling musical 
    instruments, which are only used for haraam purposes. In that case it is not 
    permissible to sell them, because that usage is haraam, and whatever it 
    haraam, it is haraam to sell it. End quote. 

    Al-Sayl al-Jaraar, 1/485-486 

    See also question no.
    105325 and
    125779 

    If someone learns this knowledge from you and you do not know 
    what he will use it for, or he appears outwardly sound, then he uses it for 
    something haraam, such as setting up a network for a pub or bar, or 
    riba-based transactions, his sin is on him and you will not be affected by 
    his sin. 

    And Allah knows best.