One of the stipulations of the Bloemfontein (1854) and Sand River conventions (1852, Art. 4) with the Free State and Transvaal respectively was that slavery will be outlawed. The continued use of apprenticeships, or inboekelingen, of black children however did point to a continuing traffic in slaves. The more distant northern parts of the ZAR (Transvaal) were a haven for this into the 1870s, as well as a broader traffic in captive people from the north and areas of the Zimbabwe Plateau leading out to Mozambique--with Boers, other Europeans and Eurafricans, and some of the local polities all taking part. (JCA Boeyens describes this in magnificent detail in his Verhoudinge tusse die Venda en die Blankes, 1864-1869.) President Pretorius famously suffered a scandal when he received 'servants' as a gift from the north, but slavery was expressly not allowed in law. However, many of these servants remained isolated from their origins their whole lives, even though the flow collapsed in the late 1870s during the first British annexation period. So there was a practice of slavery in a de facto sense for decades after it was officially outlawed within the Republics and by treaty with Britain. I'm looking up the clause in the Free State Grondwet (constitution) prohibiting it, but it's Article 10 in the ZAR's.

[edit: I'm not sure about the Republic of Natal--I'll look it up when I'm back from campus.]