Lost and Found
Answer: This question was posed to Rabbi Yitzchak Zilberstein, the rav of the Ramat Elchanan community in Bnei Brak. His response was that it is wrong to accept payment for returning a lost object even if the owner offers such a reward. A Jew returns what he has found because he is so commanded by the Torah and it is improper to demand or accept payment for doing a mitzvah.
When placing that "lost and found" ad, he suggested writing that the finder returning the lost object will receive his reward from Heaven.