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One who wears non-kosher Tefillin, even accidentally, has not received the Mitzvah of donning Tefillin. Additionally, one does not get the Mitzvah of donning Tzitzis if his Tzitzis is pasul, even unknowingly.
Rabbi Eli Teitelbaum says that similarly, one has not fufilled the Mitzvah of Tzedaka by giving to a fraud. He proves it from Bava Basra 9b, in which Yirmiyahu prays that the wicked give charity to frauds so that they wouldn't receive reward for it.
How then can one be sure that one is actually giving 10% of one's income for the Miztvah of Maaser? After all, it is very possible that some of that money is not being given to genuine poor people.