A verb is surely omitted here. What verb is it, then?
Think about what you could do with a in quotation marks. You could either _**say**_ it or _**hear it said**_ to someone and that is about all you could do with a , isn't it?
(Of course, you could **write** it but writing on someone's back would not be too romantic, would it?)
So, the omitted verb must be a form of []{} together with a quotative particle .
Normally, you say to someone face-to-face, but this is a song so, the author wants to say "saying to your back". The guy/girl is already leaving. All you (the speaker) can see is his/her back now.
> []{}[]{} ≒
>
> /
The is needed because you are saying good-bye " _ **to**_ " a person's back.
= "you, who is leaving"
> Is it (1)"You leave and say farewell to me" or (2)"You leave and i say farewell to you"?
(2), of course.