In а writing signed by bоth pаrties оn December 1, Mike the Mechаnic agreed tо buy from Easton, an engine manufacturer, a gasoline engine for $1,000, with delivery on the following February 1. Through a secretarial error, the writing called for delivery on March 1, but neither party noticed the error until February 1. Before signing the agreement, Mike the Mechanic and Easton orally agreed that the contract would be effective only if Mike the Mechanic notified Easton in writing not later than January 2 that Mike the Mechanic had arranged to resell the engine to a third person. Otherwise, they agreed orally, “There is no deal.” On December 15, Mike the Mechanic entered into a contract with an engine collector to resell the engine to the collector at a profit.On December 16, Mike the Mechanic notified Easton by telephone of the resale agreement with the collector and explained that a written notice was unfeasible because Mike the Mechanic’s secretary was ill. Easton replied, “That’s okay. I’ll get the engine to you on February 1, as we agreed.” Having learned, however, that the engine had increased in value about 75% since December 1, Easton renounced the agreement on February 1.If Mike the Mechanic sues Easton on February 2 for breach of contract, which of the following concepts best supports Mike the Mechanic’s claim?