Entering France on German issued Schengen Visa (South African passport) [duplicate]

The fact that you did not mention your side-trip to France should not be a problem as such. The details of your application will not be available to the border guards anyway.

What could theoretically be an issue is going only to France with a German visa as it would look like you tried to circumvent the rules and to prevent having your application processed by the French authorities (see also Does a multiple-entry Schengen visa allow visiting other Schengen countries later, without going via the issuing country?, Schengen visa requirements and "validation" and Can I travel to Spain using a Greek Schengen visa even if I don't want to stay in Greece at all? for more information). So you might have to explain that you will in fact go to Germany afterwards but even that might not be necessary in this case (because I think French border guards are not the most fastidious and going through France on the way to Germany is pretty common; by contrast, showing up in Germany with, say, a Maltese visa could invite a bit more scrutiny).

What you do need to check is not only that your visa will be valid on the dates you want to be in France and Germany but also that you will not exceed the maximum authorized stay and that you have enough “entries” in the Schengen area for your trip.

Concretely, your visa should look more-or-less like this:

The number of entries is on the third row. If it says “MULT” like in the picture you are OK (“2” should be enough as well, depending on your itinerary). The maximum stay is on the same line, further to the right. “XXX” means at most 90 days in any 180-day period so that would be OK as well. If you have something like 20 or so, you have to count the days but it should be enough. If you only have exactly 14 days, that's not enough for two weeks in Germany plus 3 days in France.