Just another example of a person who lacks accountability and can't except the consequences for his/her actions. As Ken stated above this was the sellers mistake and he should deal with the consequences. Judging by his response to you it seems as though he isn't understanding or accepting that. I don't see an issue with asking a buyer for the difference as long as it's done in a professional, respectful manner knowing very well that the buyer has no obligation in rectifying the issue.