Infact, we say "давай" or "давай, пока" just to say goodbye. It doesn´t have any other meanings here. It´s used in colloquial speech, is quite informal and is used to sound in a natural way.
The etymology could be probably like that: "Давай" also means let me (you, us, him, her, them), so "давай, пока" was originally someting like "Ok, let me go (I have to leave now), bye".
Russian Language Teacher.