Christ spoke those words as He had completed the work of redemption, thus defeating Satan in what Satan no doubt believed was his hour of greatest triumph. As such, having Aslan speak that phrase after killing the White Witch seems perfectly legitimate to me. And, as others have pointed out, he couldn't have said it at the Stone Table, because his mouth was tied with cords, and anyway it wasn't finished, not in the Narnian sense, until he was actually killed.