Kosher Money

The accumulation of money for its own sake has always been considered a form of idolatry, a worship of Mammon. The lust for money is considered a very dangerous passion that has no self-imposed limits. Whoever is caught up in it can no longer see anything else in the world, certainly not his fellow human beings. The halachic regulations dealing with money matters are more numerous and more complex than those dealing with many other matters, considered central tenets of Judaism. Just as their are obligations concerning kosher food, so there are even greater obligations concerning kosher money.

(Rabbi Adin Steinsaltz)