Getting beyond an image of the Apartheid SA Goverment with the nuance of a Spitting Image skit, who exactly in that Government would want Slavery to be introduced?
I am serious and that isn't a rhetorical question. If anyone can find a single scrap of evidence that a member of the National Party in a position of relative power actually spit-balled this idea seriously, then it's something we can take seriously. But beyond that, it's a awful WI because it relies on the Apartheid Goverment being this faceless torrent of pure undiluted evil that's basically the Draka, as opposed to a bunch of greedy, racist, violent dickheads who just wanted to cling to the power they had and their parents had and their parents before them had, and did so through violence and segregation.
Honestly, given how Marius is from around these parts, I trust his judgement that a PoD for 'the Apartheid Goverment reintroduces Slavery' doesn't actually exist and would require a lot hand-waving and leaps of logic, at least within the confines of what the OP provided. Wage slavery or private individuals human trafficking, maybe, but as an official Goverment policy, it's not plausible.