All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
It is permissible for the Muslim to work in charities and humanitarian organizations that are affiliated with non-Muslim countries as long as this work does not involve engaging in a forbidden act or helping others do what is prohibited. In principle, it is permissible for a Muslim to hire himself out to a non-Muslim to work in a permissible job unless the job is to work as a non-Muslim's personal servant. For more benefit, please refer to Fatwa 81686.
The Kuwaiti Fiqh Encyclopedia reads, “Muslim jurists agreed that it is allowable for a Muslim to hire himself out to a non-Muslim to do a specific, well-defined job such as tailoring a garment, building a house, cultivating a land, and the like. This is because ‘Ali hired himself out to a Jewish man as a laborer to draw water from the well in return for a date per bucket (to irrigate the Jewish man’s orchard). The Prophet, sallallaahu ‘alayhi wa sallam, was informed of that and he did not object. Another reason for making it permissible is that the Muslim worker who hires himself out to do a specific job which is described on liability (i.e. the ultimate goal of this hiring is to acquire the resultant usufructs) may get someone else (a third party) to do that specific job for which he was hired. Muslim jurists also agreed that it is impermissible for a Muslim to hire himself out to a disbeliever to carry out forbidden tasks such as winemaking, shepherding pigs, and the like. However, scholars held different opinions regarding a Muslim working as a servant for a disbeliever.”
We advise the Muslims working in these humanitarian organizations to hope for the reward of Allaah for helping the poor and needy. The Prophet, sallallaahu ‘alayhi wa sallam, said: “There is a reward for serving any animate (living being).” In another Hadeeth, he said: “The honest Muslim trustee who does what he is commanded to do, pays fully what he has been ordered to give with a good heart overflowing with cheerfulness, and gives it to whom he is ordered, he is one of the givers of charity.” [Al-Bukhari and Muslim]
We also advise them to invite those people to Islam if they can.
There is no doubt that a Muslim is commanded to be fair and truthful even with respect to non-Muslims. It is impermissible to accuse organizations of carrying out missionary work without clear evidence.
Allaah Knows best.