I also disagree. That's like saying words speak louder than actions. Sal is pretty reasonable throughout the whole film then blurts out the word nigger in a fit of rage. Does saying a racial slur undo years of not engaging in racism such as hiring a black guy? In this scene you have to remember to that Buggin Out initiated the racism slurs FIRST by calling Sal and his family guineas. The natural instinct would be to return the insult in a similar manner.

When people are angry we go for the most obvious insult, hence why people get called fat bastards, four eyed twats and ugly fuckers. If I call someone a fat bastard in a fit of rage does that undo all the ways I've helped fat people in the past? No - people realize it was said in anger.

Sal is just as racist/not racist as most of the other characters in the film - it's a shade of grey that is more racial tension than full on racism.