This isn't as black & white technical as you're making it. State of mind matters, like a lot. At first, both of them were just bantering back & forth. Then, she raised her fist in a threatening manner and gave notice that she was going to "fkn punch you in the face, btch." Now, that's inarguably a threat & assault, full stop; you're not getting around that. Next, he said you don't have the balls- based on his casual demeanor, he wasn't taking her threat seriously and thinks they're still just bantering back & forth with there being no "actual" threat. She then asked if she could slap him, and again he said go ahead you don't have the balls, in a manner that suggests he doesn't believe she'll actually do it. That's not him literally consenting to being slapped.
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