Here, is 'znajomym' acting as an adjective in instrumental, unlike in English where it's a noun?
It's both a noun and an adjective, so in this context (an acquaintance) it's a noun and behaves so - therefore it goes into Instrumental.
The "negation = Genitive" rule cannot be extrapolated too far. It works only when the positive sentence used Accusative. Otherwise, the case just stays. (which kinda means that it works also when the positive sentence used Genitive :P)
AH, I see now! An acquaintance- someone you're familiar with: "Znajomy", and "I know" - to be familiar: "Znam" :D
Even more of a complication is that in English one says 'this gentleman is not OF my acquaintance'. It deceived the computer which would not accept the 'of'.
I attempted an acquaintance previously, would not fly. Pretty much means the same.
Computer again, it was not accepted, I thought that was peculiar. Judging by what mine less than a year old doeas to me at times, I can undestand. In the middle of a lesson I lose my screen and go to log on mode and nothing will bring it back.