It did say on the original page that I linked (it has since been changed) that AFTER the birthday celebrations are over, the game would be tradable.
The game page itself has been edited as well - When I downloaded the game, I had to provide my full log in details etc., so maybe they have changed their minds about the trade in.
The deal is still shown on their blog -
http://blog.greenmangaming.com/2012/05/birthday-gift-from-green-man-gaming/?utm_source=social
"And the birthday gift you’re all waiting for? You can get Disciples II: Gold Edition from Green Man Gaming for free for the next 46 hours. Is it not your preferred genre? After Saturday you can trade it back in against other purchases to get money off! Find out more here."