When Schultz says farewell to Candie, it signifies the end of a fairly subtle character arc.
"Since what auf wiedersehen actually means is 'Until I see you again,' and since I never wish to see you again, to you sir, I say goodbye," he says.
Then, when Candie insists he shake hands before leaving, King finds the idea so repellent he chooses to shoot him instead.
That's despite the fact Schultz actually shakes Candie's hand when he first meets him, even though he's already described him as a despicable man.
Also, he says auf wiedersehen when he shoots the regulators… and we're pretty sure he didn't want to see that bunch of cowards again.
So, despite some critics claiming he doesn't change throughout, Schultz does go on a journey.
He starts the film as a liberal, with an obvious distaste for slavery. But he doesn't give Django his freedom immediately, stating that he'll keep the slave / master dynamic in place, because it's useful to him.
However, the atrocities he witnesses over the course of his travels with Django means that he is unable to offer even simple pleasantries to a slave-owning plantation master he'd previously bowed before.
It's a neat arc, and one which is linked to the most moving moment in the movie.
When Django kisses his own hand, lays it on Schultz's head, and says "Auf wiendersen." Only the hardiest members of the audience would have failed to shed a tear.