Just maintain a professional attitude even if this guy tries to get a rise out of you. You have already offered to refund the alleged shorted amount, that is reasonable and fair and the SR admins will see that when they read your correspondence with the buyer. If they are alluding to using the forums to blackmail you it will not help their cause much. Make your proposal to refund the alleged missing amount and let it run through until an admin gets to it, I'd be very surprised if they didn't accept your proposal.