A deal has been reached between the owners, a developer and two nonprofits on a valuable piece of property in Seattle's Central District. Lake Union Partners has agreed to buy the old Midtown Center property on 23rd Avenue and East Union Street for $23.25 million. The property is one of the last full blocks of undeveloped land in the city and lies at the historical heart of Seattle's black community. Lake Union Partners plans to sell 20 percent of the block to land conservancy group Forterra,