I work as a guard in a building that belongs to a money-changer, for which I am paid a salary. I worry to myself about the doubtful nature of the money that he pays me. Is there any harm in me taking it?
If this money-changer deals in Riba (usury/interest), you should stop working for him and look for alternative work where the money you get paid contains no Riba. It was authentically reported that the Messenger of Allah (peace be upon him) said,
“Leave that which makes you doubt for that which does not make you doubt.”
(Part No. 15; Page No. 8) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.