tvor: Ma’aser K’safim, B’pashtus, is an Chiyuv D’rabannan.
Yitzchokm: I’m not denying Dina D’malchusa nor the fact that it applies more widely than some people think (I think we’ve had this discussion before). I’m just saying that American tax law is not the same as the Chiyuv of Ma’aser Kesafim.