Terumah 5773

The Parsha begins with Hashem telling Moshe to take donations from Bnei Yisroel for the building of the Mishkan. This seems to say that Bnei Yisroel was forced to donate. But didn't they give with a "nediv lev," a motivated heart? R' Moshe Feinstein writes that when one gives tzedakah he shouldn't do it because Hashem told him to; he should give because he himself wants to give. Using his yetzer tov a person implants in himself a feeling of giving tzedakah out of his own good intentions.