A non-profit organization has agreed to buy Killingworth’s 300-acre Deer Lake property and preserve it as open space, ending months of concern that the property would be sold to a private developer.Pathfinders, Inc., which operates the Deer Lake Camp and Wilderness School, announced on a website Thursday that it had signed a contract with the Connecticut Yankee Council of the Boy Scouts of America to buy the camp for $4.75 million. The agreement covers the property, its buildings, and other improvements. The closing is scheduled for Sept. 15.