Nowadays, statistics show that more than 90% of renewable energy systems are connected to the European distribution networks inducing technical issues and costs increase for network reinforcement. However, network reinforcement may not be the most efficient solution from both energy and economic point of views. Therefore, new strategies need to be implemented in order to efficiently manage both grid infrastructure and renewable resources. These strategies should use flexible passive and active users, such as demand-response and storage systems. Hence, in this paper, a contract-based framework for capacity allocation of microgrid's flexibilities is discussed to implement an active management of distribution grids. Moreover, an optimization model for day-ahead congestion management scheduling based on generation and flexible load flexibilities in microgrids is proposed. At the end, the effectiveness of the proposed model for both distribution system operators and microgrids is tested with a case study and simulation results are reported.