Exactly how do you know for a fact the song was completed? Just because it was supposed to be on Thriller doesn't mean it was finished. Michael could have been working on the song and when he found out it couldn't be put on Thriller, he stopped working on it. On top of that, BTM has crowd noise in it from the Bucharest Dangerous concert. If the song was complete, why did they are this?
Behind The Mask, was chosen and finished. Then he found out he couldn't release it due to legal, as I said.
Going by interviews, and sources, it sounded like it was completed.