In the Name of Allah, most Compassionate, most Merciful

It is a very common mistake for the Muslims of today to work for the disbelievers.  The few verses of Quran will clear up this insha'allah.

Surah Tawbah 9:23. O you who believe! Take not for Auliya your fathers and your brothers if they prefer disbelief to Belief. And whoever of you does so, then he is one of the Zalimun (wrong-doers, transgressors , etc.). 

Auliya in Quran and Shariah generally means; having to do anything with in any way.  So it includes, working for them or having them work for you.  Also...

9:29. Fight against those who (1) believe not in Allah, (2) nor in the Last Day, (3) nor forbid that which has been forbidden by Allah and His Messenger (4) and those who acknowledge not the religion of truth (i.e. Islam) (even) among the people of the Scripture (Jews and Christians), until they pay the Jizyah with willing submission, and feel themselves subdued.

So it is haram to work for disbelievers, and the money is haram as well.  Normally some Muslims use to work/trade with disbelievers in early days of Islam until these verses were revealed.