Q. If this (the tzedaka given to a collector who turned out to be a fraud and a con) was part of my maser money, do I have to compensate or does it count?

A. Horav Shlomo Miller’s Shlit”a opinion is that you do not have to replace the ma’aser money, as this is similar to one giving his terumah or ma’aser rishon to an Yisroel, and informing him of the nature of the gift. It places now the onus on the recipient to convey the produce to the proper Cohen or Levy. In this case the fraudulent collector has now the obligation to pass on the tzedaka to the poor.

Rabbi A. Bartfeld as revised by Horav Shlomo Miller Shlit”a