I know it wasn’t ever answered in the TV series. It might be answered in the comics, but as I haven’t picked those up yet, I can’t help you there. However, the group that Angel signs the paper for are the same group that Angel and the gang were trying to destroy at the end of s. 5 (Wolfram and Hart), so I would assume if they went away, the contract wouldn’t matter anymore. But…W&H are huuuuge. Also, the prophecy is sort of implied, throughout the series, as something that happens waaay in the future, after Angel (or Spike) has saved the world and himself many times over. So, maybe by the time Mal and the rest of the Firefly gang catch up with one or both vampires, the prophecy will have come true! :P Sorry I can’t help more!