South Portland City Manager Jim Gailey announced late Thursday that he has signed a contract to sell the former National Guard armory to a Topsham development company. Under the purchase-and-sale agreement, Priority Real Estate Group would pay $700,000 for the building at the foot of the Casco Bay Bridge. That’s $50,000 more than the city paid for it eight years ago. The developer’s plan calls for converting the armory into a convenience store and cafe with a fueling station behind the landmark building at 682 Broadway. “We are pleased to be working with Priority Real Estate,” Gailey said in a written statement to the Portland Press Herald.