The (Ba,Sr)FeO3-delta system is known for its strong tendency for oxygen and vacancies to order into several forms including fully ordered pseudobrownmillerites, hexagonal perovskites with segregation of the vacancies in particular anionic layers and low deficient (pseudo)cubic compounds (generally delta < 0.27, Fe3/4+). We show for the first time, using a simple chemical process, the easy access to a large amount of vacancies (delta approximate to 0.5, Fe3+) within the room-temperature stable tetragonal (pseudocubic) Sr0.8Ba0.2FeF similar to 0.1(O,F)(similar to 2.5). The drastic effect of the incorporation of a minor amount of fluoride passes through the repartition of local O/F/square constraints shifting the tolerance factor into the pseudocubic range for highly deficient compounds. It is stable up to 670 K, where an irreversible reoxidation process occurs, leading to the cubic-form. The comparison with the cubic oxide Sr0.8Ba0.2FeO similar to 2.7 shows the increase of the resistivity (3D-VRH model) by two decades due to the almost single valent Fe3+ of the oxofluoride. In addition, the G-type magnetic ordering shows relatively weak moment for Fe3+ cations (M-Fe approximate to 2.64(1) mu B at room temperature) attributed to incoherent magnetic components expected from local disorder in such anionic-deficient compounds.