On Tisha B'Av, the custom is to refrain from greeting people. Even a brief "hello" is to be avoided if possible. Does the same restriction apply to saying "good-bye"? Or is that not considered a greeting for this purpose?
No, Rav Shmuel Kamenetsky in sefer Kovetz Halchos 232:13 that saying "shalom" is forbidden but to say "see you later" or "good night" before one goes to sleep is not a problem, but "good morning" is. See footnotes in the book if you have access to it.
I heard from my Rav that even "see you later" or "good-bye" is forbidden for a mourner and on Tisha BAv.