Many authorities explain that according to the “dry” letter of the law there is no obligation to tear up the stamp. The onus was on the postal service to “stamp” the stamp.
However, I personally think that the correct thing to do is not to use the stamp again. There is a famous story told about the renowned Rabbi Yaakov Kaminetzky. Before he came to America, Rabbi Kaminetzky was the rabbi in a small town in Eastern Europe. A Jew asked him if he was obligated to return extra change the postmaster had been given him by mistake. Rabbi Kaminetzky ruled that the Jew should return the money, which he did. The postmaster was so overwhelmed with the honesty of the rabbi and the Jewish community that he almost single-handedly saved the entire community from the Nazis when they swept through the town.
