It says "buck" in the subtitles, so I assume that was the intention of the dialogue.
Besides, I think buck makes way more sense considering he isn't getting a bump in pay, just an extra buck due to her tip.
Bump makes just as much sense as buck to me. I never turned the subtitles on, but if you listen closely it's bump. Not that it matters either way.
I wanted to see that scene recently as I said, so I started watching season 4. I'm almost finished it already and I guess I may as well start at season 1 after this and then skip 4 on another rewatch. This will probably be a half dozen.
Edit: one flaw I noticed in season 4 is when they go back to the scene where Herc fired the nail gun, it's clearly a different place. Initially it was on a curved curb, maybe near an intersection, when they go back it's in the middle of the street with a straight curb. I only point it out because normally The Wire is so careful about stuff like this.