A nice illustration of "negative value" is the Nick Punto plus throw ins deal between the LAD and Boston in 2012. LA agreed to take on one player who had a huge contract but really could not even take the field anymore in Carl Crawford, and another guy who was still owed a lot of money, probably more than what he'd get in the open market at the time, but who could contribute to a contender in Josh Beckett, as well as a useful, properly priced veteran utility player in Nick Punto, but in order for the Red Sox to get back a bunch of lottery tickets that ended up never contributing at the major league level, they had to include a high dollar contract with surplus value in an MVP-level performer in Adrian Gonzalez. The overwhelming reason Boston did that deal was to create CBT room to go out and sign the vets who made up their 2013 team (the other reason was supposedly attitude adjustments).
Beckett's case is the closest to Corbin and Stras right now. I believe he cleared August trade waivers prior to the deal (I think Gonzalez had been claimed by LAD). To a contender, the extra value a stronger roster provides going into the playoffs might make them drive a less hard bargain for someone they perceive as short term help, but even then, they insisted on extra back. Boston was willing to put Gonzalez in the deal only to get rid of Crawford's contract, too.
With Corbin, there might be takers, but it is going to take some sweeteners to get anyone to take his contract on. I guessed $45MM of value to knock his contract back down to closer to 4 years, $60MM remaining. I may be a little high on that for a contender if he's pitching well, and if half this year is already paid out. As for Stras, he really needs to show he's healthy because he's impossible to price right now.