Ohner and Planner signed a detailed writing in which Planner, a landscape architect, agreed to landscape and replant Ohner's residential property in accordance with a design prepared by Planner and incorporated in the writing. Ohner agreed to pay $10,000 for the work upon its completion. Ohner's spouse was not a party to the agreement, and had no ownership interest in the premises. For this question only, assume the following facts. At Ohner's insistence, the written Ohner-Planner agreement contained a provision that neither party would be bound unless Ohner's law partner, an avid student of landscaping, should approve Planner's design. Before Planner commenced the work, Ohner's law partner, in the presence of both Ohner and Planner, expressly disapproved the landscaping design. Nevertheless, Ohner ordered Planner to proceed with the work, and Planner reluctantly did so. When Planner's performance was 40% complete, Ohner repudiated his duty, if any, to pay the contract price or any part thereof. If Planner now sues Ohner for damages for breach of contract, which of the following concepts best supports Planner's claim? 
Substantial performance. Promissory estoppel. Irrevocable waiver of condition. Unjust enrichment.