6G networks are predicted to provide new prospects for Smart Cities and Internet of Things (IoT) applications because of their global seamless coverage. Therefore, to fulfil the growing need for huge data rates for 6G and greater applications, network capacity must be enhanced. As a result, there is an increase in spectrum demand. Only by successfully sharing existing spectrum and avoiding spectrum underutilization will the increased demand for cellular services be addressed. As a result, for 6G to achieve considerably enhanced network capacity, efficient spectrum management systems must be developed. As a result, maintaining 6G's predicted huge network capacity in such as heterogeneous environment necessitates the shared exploitation of available spectrum resources through dynamic coordination across device and network, which can be accomplished by incorporating SDN into 6G networks. Due to the increased speeds and reliability of 6G networks, users may have to pay more for energy, to overcome these issues, In this paper, a novel proposed a 6G HetNet spectrum management system based on HSA and Smart Contracts. HSA harmonizes network operation by spreading local decision-making and network-wide policy-making processes between BS and the SDN controller, correspondingly, to relieve any possible controller scalability and latency difficulties. And, to tackle the intricacies of service-level agreements, leverage blockchain's smart contract technology, which allows for automation and trustworthy, transparent radio spectrum negotiation among several parties. This proposed solution is dependable, scalable, and implementable, as seen by the results.