Complex sentence: Sarmiento re-joined his previous club Argentinos Juniors on loan until June 2007 before joining Larissa in Greece on a free transfer on a two year contract plus one year extension option.
Split into simpler sentences: Sarmiento re-joined his previous club Argentinos Juniors on loan until June 2007. As a free transfer, Sarmiento joined Larissa in Greece for two years plus one year extension option.