He tried to back out of the Disney sale, but the fee was going to be something like 500m to back out of the sale.
They reached a point in negotiations where Disney had certain papers signed-- but no final pricetag on the deal. So Disney begins corporate structuring and preparation for the impending close of the deal, which costs Disney money. Investors start moving differently, the board, the executives start working on initiatives for the new studio acquisition, and so forth. So Iger threatened to sue Lucas Film for about half a billion if George didn't sign off and do what he said he would.
The rest of the details I can't remember. But it wasn't like George was thirsty to sell. The sale was dragged out, protracted in terms of back-and-forth, and George kept trying to change the terms, leading to Iger to issue the threat and just muscle Lucas Film away from him. Granted, George is known to diddle and dally. A cold-feet prone type of guy. Iger was a shark. Sharks eat. He knew what he was getting into by entering advanced talks with Disney.
This is the basis of George's "White slavers" comment. He's still a little salty at how Iger forced the sale-- but he did it to himself.