Now I get it. From this satellite view, it becomes apparent that Rezko didn't buy an adjoining lot to Obama. He just paid for part of the land that is really Obama's yard. Look at the hedge, it is continuous across both properties.
That is Obama's house in the center of the frame, and Rezko's "lot" just below it. A "lot" too small to build on. So it will always be Obama's private park-like buffer to Hyde Park Boulevard.