Murrey manufactured and sold pool tables. He was approached by Madsen, who had an idea for a kind of electronic pool table that would light up and make sounds like a pinball machine. Madsen made a $70,000 deposit on an order for one hundred tables but then encountered difficulties and notied Murrey that he would be unable to accept delivery of the tables. Murrey broke the tables up, salvaging materials worth about $15,000 and using the rest for firewood. The evidence was that the tables, if completed by Murrey, could have been sold for $45,000 as regular pool tables. Madsen gets his deposit back less expenses incurred by Murrey. But what principle affects Murrey's measure of damages, his right to claim expenses incurred