By arbitraging among consumer comfort margins, buildings energy consumption can be changed by providing flexibility to grids. To manipulate the buildings energy consumption, a new contract-based approach to for multi-zone building heating, ventilation and air-conditioning (HVAC) systems is proposed. The approach includes the real-time markets by changing buildings optimal consumption pattern based on triggers sent by the aggregator. Also to decrease the energy consumption of buildings, the user is allowed to select the time-slots and rewards are provided to the user for aggregating flexibility. The aggregator bundles flexibility from the buildings at different time-slots and sells in real-time markets. The idea in aggregator's problem is to maximize aggregator's profits by selling flexibility in real-time markets (RTM) while ensuring the provisioning of flexibility from the buildings through incentives. To address this problem, we formulate it as a distributed optimization problem and then provide a method to solve it which provides good scalability, a requirement for large commercial buildings with multiple zones to participate in RTM. We illustrate the scalability and performance of the contract-based approach and solution technique in a building with 200 zones. Also, user participation based on their time-preferences is included in the proposed optimization. Finally, a scalable technique is shown which can be adopted in existing building automation systems.