Given that they've already neg'd you and probably already left a low DSR, don't give them the satisfaction of a refund. If it's blatantly obvious that's what they do, they can't make things worse for you. Chances are, even if you asked, they wouldn't revise the feedback anyway. Tell them to take a hike, you're not falling for their scam.

Without putting their actual username on here, which you're not allowed to do, any hints of who this person may be? 

Don't forget to add them to your blocked list.