Void doesn't want Femto to go back to the world of men to complete his ambition, rather wanting him to do his job as God Hand and Femto, naturally doesn't want that.
Could go either way, considering the two interpretations of the God Hand. On one side, the God Hand seem to treat Femto as being quite a special member. On the other hand, Void is their leader. I'm guessing that for now, Void > Femto, though Femto will likely be the highest level God Hand member by the end of the series.