So, if this piece of land was actually a gift to Obama (perhaps not on paper, but as far as common-use is concerned,) wouldn't that be against the law or very close to the edge of existing law?
Yes. It's quite similiar to what has Sen. Stevens of Alaska under federal investigation. In that case, known political fixers in the state used a series of "sham" transactions to pay for major upgrades to the Stevens home (adding a second story).
Why do you think Obama is trying so hard to distance himself for any work done for Rezko? Rezko clearly "scratched Obama's back". If it were to come to light that Obama "scratched Rezko's back" in return, we would have very serious suggestions of Chicago-style corruption.