In a writing signed by both parties on December 1, Kranc agreed to buy from Schaff a gasoline engine for $1,000, delivery to be made on the following for $1,000, delivery to be made 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, Kranc and Schaff orally agreed that the contract of sale would be effective only if Kranc should notify Schaff in writing not later than January 2 that Kranc had arranged to resell the engine to a third person. Otherwise, they agreed orally, "There is no deal." On December 15, Kranc entered into a contract with Trimota to resell the engine to Trimota at a profit. For this question only, assume the following facts. On December 16, Kranc notified Schaff by telephone of Kranc's resale agreement with Trimota, and explained that a written notice was unfeasible because Kranc's secretary was ill. Schaff 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, Schaff renounced the agreement on February 1. February 1. If Kranc sues Schaff on February 2 for breach of contract, which of the following concepts best supports Kranc's claim? 
Substantial performance. Nonoccurrence of a condition subsequent. Waiver of condition. Novation of buyers.