This paper examines the job exchange between parallel compute sites in a??decentralized Grid scenario. Here,the local scheduling system remains untouched and continues normal operation. In order to establish the collaboration and interaction between sites in a Grid context,a middleware layer that is responsible for the migration of jobs is supplemented. Independent users are assumed to submit their jobs to their site-local middleware layer, which in turn can request jobs for execution from alien sites. The simulation results are obtained using real workload traces and compared to the performance of the EASY Backfilling??algorithm in an equal single-site scenario. It is shown that collaboration between site is beneficial for all high utilized participants as it is possible to achieve shorter response times for jobs compared to the best single-site scheduling results.