Section 25 of the South African Bill of Rights, signed into law by Nelson Mandela in 1996:
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the right to property, limited in that property may only be expropriated under a law of general application (not arbitrarily), for a public purpose and with the payment of compensation.
In January, South African President Cyril Ramaphosa signed into law a bill that allows land expropriation without compensation in cases where it is deemed "just and equitable and in the public interest".
Trump is wrong in that they haven't done it yet, but they are certainly trying to. The only debatable aspect is whether the constitution or the new law is more just.
It's also important to note that this isn't only about white people, Zulu land is also a target for expropriation without compensation