I didn't bother to read the thread but huh, never put much thought into what he said after "take it".
Be it "keep it", "Even", or "Heathen" I feel like regardless it's in character. Though I kind of like the idea of Greyback calling Harrigan a heathen given the Predator's devil name and how it operates. Like he gets the trophy out of some degree of respect but Harrigan isn't one of them if that makes sense so he doesn't get City Hunter's head for one reason or another and gets an alternative trophy. Though I'm pretty sure he just tells him to keep it.