Ani Ma'amin (I believe) written on the way to Auschwitz - An amazing story

Here's Chilik Frank playing a version of Ani Ma'amin (I believe). In the first two minutes, he tells the story of the tune, which was written by a Mozitcher Chassid named R. Azriel David Fastag HY"D (May God Avenge his blood) on his way to the death camps. R. Fastag offered half his share in the World to Come to anyone who managed to take his composition to his rabbi, the Mozitcher Rebbe, who by