On Monday, Billy receives an offer from Andrew to buy Billy's house for $500,000. On Tuesday, Billy mails Andrew a reply, "I'll sell it to you for $600,000, and not a penny less." On Wednesday, Billy reconsiders so he mails Andrew the following note: "I accept your offer for $500,000, the house is yours." On Thursday, Andrew receives Billy's first note. On Friday, Andrew receives Billy's second note. What result?