Nope. UNILATERAL option.
The Bulgarians current deal expires at the end of the season but United will trigger a unilateral option this week to extend his contract by a further year to prevent the player from leaving on a free transfer in the summer.
And Berbatov would have been aware of this when he originally signed for United.
It's not like United have made up a new contract clause.
This would have been in his contract when he signed for United.