Answer: Apartheid.
The implicit issue that represents a major conflict between white and black South Africans is the APARTHEID.
This is reflected by Fugrad, writer of : Master Harold and the boys. where Hally and Sam symbolize with their conflicts this practice that did not allow black and white South Africans to live together in peace and harmony.
Fugrad reflects on Hally's attitude when spitting at Sam, the difference imposed by this absurd practice.
However, the attitude of Sam was the best, to ignore this offense, and treat it with great cordiality.
The message it conveys is totally clear: to live in harmony and without distinction of race or color.
The minor conflict of Hally and Sam is intended to reflect the discrimination suffered by black South Africans during the apartheid period, condemning them to live apart as if they were empires only because of the color of their skin.