After the initial shock of "Holy Hell!" I'd probably wait a few days for a /tell. If he/she acted like a total *** or *** about it, then I'm 4mil richer with a clear conscience. However, if they actually made sense and seemed kind enough, yeah, I'd probably refund it... Well, most of it, anyway. >.>
EDIT: In my paranoia, it looks super like an RMT transaction >.>