I found this online:
“Each person who passes to be numbered, from age twenty, shall give a half-shekel, from a shekel of the Temple,… a donation to HaShem.” [30:13]
The Medrash says, “Rabbi Meir said, the Holy One, Blessed be He, brought out from beneath his throne of Glory a coin of fire, and showed it to Moshe, and said to him, like this shall they give.”
The Noam Elimelech explains why HaShem showed Moshe a coin of fire. When handling money, he explains, one should remember that a coin is very much like fire. Just as fire can burn and destroy, but can also warm, and cook, and serve in other valuable ways, so too a coin. If one gives a coin for a good purpose, for charity or kindness, then it is very valuable and helpful to a person; but if a person uses his money improperly, then the same coin can burn and cause serious harm.