"Uhuh." Buffy's skeptical expression says it all - not that she's seriously annoyed at him, of course. She's evidently just messing with him. "So you're not her pet? I'm just checking."

She blocks the punch with ease, bringing one arm up to stave off the blow, and swinging a return punch with the other. Naturally, she can tell he's holding back, and she is too, going for somewhere near her regular Slayer strength.