You shouldn’t be greeting the other person at all unless he is a very close friend or an immediate relative. Anyone else, you are just supposed to stare them up and down as you go past, which is the same for shabbos and motsoai shabbos. If the other person greets you first, you just act puzzled and say, “ich kenst dir fun vainekkin?” Again, no difference whether its shabbos or not.
If it happens to be a close relative or friend, you can call his house later and explain what happened.