Sorry, I geuss I should've checked my facts, instead of relying on memory. How or why the British seized it in the 1790s isn't really the issue, it's to motivate them not to buy it/or the Dutch to offer to sell it to them after Amiens. That the Dutch are the ones holding an empire here. As said, Britain will probably take OTL Walvis Bay (IDK what the anchorages are like further down the West Coast, so I think Walvis Bay might be the last decent harbour) or maybe they establish a base at Natal (Durban) if they want South Africa bad enough. But personally, as mentioned, any of the Dutch Indies' islands would've been more valuable (remember dawn of the 19th century no one knows about the South African gold/diamond reserves) than the Cape. So, simply let the British say that they rather want Indonesia or Suriname, and the Dutch tell them to go fly a kite, in a thunderstorm, and keep the Cape. Obviously it'll probably either be conquered by the British outright, or pass to the Bonapartean kingdom of Holland (which could have some interesting effects).
To clarify - I was referring not to the 1790s, but 1806. As far as I can tell, no sale was involved - it was simply a matter of Britain conquering the colony. Then, in the Convention of London in 1814, it agreed to return most of the Dutch possessions as of 1803, but not the Cape or the regions in South America that became Guyana.