In systems from which multiple resource providers dynamically provide computer resources over the Internet, a decentralized resource allocation mechanism is essential. In such an environment, resource providers and resource users have different requirements. Satisfying the requirements of both is difficult because there is no centralized resource broker. To resolve this issue, we propose a sufficiency-based, on-demand resource reallocation mechanism that permits reallocation of resources when a current resource user has a higher utility level than that of a resource requestor. We have implemented this mechanism and used it to allocate resources in a simulated environment. Our experimental results have shown that sufficient resources were provided to 48 percent more users by applying our mechanism as compared with a similar mechanism without reallocation.