I think we'd have to get the folks in the Giants front office highly inebriated before they would even consider taking Stewart's contract.
It really wouldn't be all that bad. We'd eat his bonus, so the Giants would only be responsible for just over half a year of his 2013 base salary. Typically, most players in the NFL don't have their base salary guaranteed (so I'm assuming neither does Stew). And if they don't think he's worth his salary, they could then release him at seasons end without owing him anything more. However, if they do want to keep him, but not at that price, they can try to renegotiate his contract.
Bottom line is it's really a low risk situation for the Giants. The question is if they would even want Stew...particularly since it's unlikely he'll see the field prior to the trade deadline so they won't know about his health. Not to mention that we might not want to let him go, or even have any interest in Nicks. But if both teams do have interest, the trade does make sense for both teams.