Well, if you check Fact Check.org the issue is is a pretty real estate deal for both of them.

Here is the gist:
Million-Dollar Mansion

As for that claim about Rezko helping Obama buy his house, well, we've dealt with that one before. The gist of the story: In 2005, Barack and Michelle Obama found a house that they wanted to purchase. The property had been divided into two parcels, one containing a house and the other undeveloped land. The owner had listed the properties separately. After considerable haggling, the seller accepted the Obamas' third bid of $1.65 million for the parcel containing the house. Tony Rezko's wife, Rita, purchased the adjoining lot for $625,000.

When the Obamas wanted to increase the size of their yard, they approached the Rezkos about purchasing a strip of the adjacent parcel. Obama told the Sun-Times that a 10-foot strip of the 60-foot lot appraised for $40,000. The Obamas nevertheless paid Rita $104,500 (or 1/6 of the total purchase price of her lot) for the strip. In 2007, Rita sold the remaining lot for $575,000 (or roughly a $54,500 profit on the overall property).

McCain's ad is worded in a way that could leave a false impression. It says Rezko "helped him buy his million-dollar mansion" by "purchasing part of the property he couldn't afford." That's true, but only because the seller wanted to sell the two parcels as a unit and the Obamas couldn't afford both. Rezko did not make a gift of any property to the Obamas. Furthermore, the fact that his wife sold her lot for more than she paid for it contradicts any suggestion that the Rezkos overpaid for their part of the deal as a way of getting the seller to lower the price to the Obamas for their part.