You have a South African Passport and surely that means, that whatever you had in your foreign passport is not needed?
Going and asking for both passports to be stamped when you leave is only going to cause headaches I would have thought.
For me, the only time I show my UK passport here in South Africa is to the airline if I am flying to a country where I intend to enter that country with my UK passport.
That ^
Don't show both, just leave on your SA passport.
And in future use your SA passport in/out of South Africa
People I have done this, I have spoken to Home Affairs and to the officials at the airport, I have got the jacket.
If your foreign passport is not registered as leaving SA it WILL be flagged as an overstay, you are therefore in SA illegally and that passport will be barred from entering SA again. Your visa doesn't magically evaporate when you get an SA passport.
NGinCreator
Call the Home Affairs number 0800 60 11 90, they will tell you to go to a DHA office
At the DHA Office you will be told to go to immigration, at immigration you will be told to take both your passports to the border post.
At the border post show them both passports they will say ok shap and let you out on your foreign passport saying just come back in on SA passport, make a note of the time and officials name just in case.
When coming back in hand over only your SA passport, they don't even blink and let you in.
If the visa you arrived on has already expired(which I'm sure it has, you had to be given some sort of stamp/whatever to get in, they don't just let you waltz through) then just use SA passport and forget about ever entering SA on that foreign passport again.