Thanks for your post, I didn't realise the extent of the debt incurred by South Africa at the time.
I was actually thinking of starting a thread with the opposite ATL, ie where the West took a much harder line against South Africa than in OTL (where lots of sanctions were quite easily broken with the implicit support of the US/UK). Of course Cold War reasons would prevent this without a significant POD, perhaps having the major black anti-apartheid groups (ANC,etc) being non-socialist or at the very least not backed by the Soviets.
Your point about the debt is interesting, as I would assume that if the West wanted to take a harder line against SA, then not being willing to loan money either directly or via the IMF would be the most effective way. Of course the SA Govt could still have monetised the deficit (printed money), but most govts are loath to do this and it would still effect their exchange rate.
By the way, were there any anti-apartheid black organisations that were also non-socialist and/or pro-Western? Did they have any significant support among the black population?
The fact was that during this time, South Africa had very high interest rates in order to attract foreign capital to the country. However, this made it very hard for people living there to get a loans & credit, hurting the economy. My father and mother couldn't buy a house when they lived there because the interest rate was something around 20%, and even a modest mortgage was unaffordable.
I imagine that if the sanctions were even stricter, the South African reserve bank would raise rates even further, stagnating the economy, rather than try to print their way out of debt.