Okay, I have changed my mind.
After listening to the edited clip a whole bunch more, he says something like “How would you like to be in the White House...
He absolutely does not say the N-word, imo.
The biggest clue here is the lack of reaction.
Okay now the question is were did the dubbed video come from.
Except that Stephie shakes his head "no" after hearing it.
The remark was whispered and Stephanopoulos does react, shaking his head "no" with his face mostly turned away from the camera.
I see no reason to conclude from this that Kantor did not say "white ni**er". They know they are being filmed. This is an important scene. Kantor let the mask slip but why not just slough it off as George does?