If you have the property under contract for 62k and you have someone willing to give you 85k then you have nothing to worry about. The person who wants it for 85k has no other chance of buying it at all without doing business with you. Be up front with him, its nothing personal just business and this is how you make out on the deal. If he has problems then you negotiate to save the deal, example drop his buying price a little to make sure he goes through with it. That way it is win/win. he gets the house he wants, you get some cash.
