The Green Knight took the scavenger's head in place of Gawain's.
The scavenger steals Gawain's sash, then is never seen again, and later the sash turns up in the hands of Lady Bertilak, whose husband is implied to be the Green Knight in disguise. Perhaps at some point he came across the Green Knight, who was supernaturally aware that the scavenger had promised to "finish [Gawain's] quest for him" and made sure to hold him to his word.
