Voyager Staff returns the create under the owner's control. So if you take a creature from your opponent with slave and exile it with staff, it would be returned to their side of the battlefield. If you use slave to target your own creature (to untap/give it haste), it would return under your control. In both instances, since the creature is changing zones, the sacrifice clause is countered.
To be clear, there is no countering of anything going on here. Slave sets up a delayed trigger for the creature it steals and Staff makes the creature a new object, so the delayed trigger can't happen at all.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules