I wasn't sure whether to post this in the bug reports forum or here; it may be intentional (in which case I've beaten the game). I tried a quick search of the board and didn't find anything on this, but my apologies if it's a duplicate. I'm sure someone else has noticed this before.
Sometimes when I trade for players and have the other team retain 100% of the salary, the AI still treats them as negative value. E.g.
Often these players are the ones that keep popping up when shopping players, and they are often, but not always, on the trade block. When it gets close to the trading deadline, I can often get three star relievers for free (100% retained, sending $1) and I can build trades that look something like "4 star reliever/3.5 star hitter (retaining X amount) + 2.5 star free player + 3 star free reliever====2.5 star reliever (retain 100%) + 2 star 3B (retain 100%) + 2 star reliever (retain 100%)."
Interestingly, you can then flip these players to another rebuilding team, who puts the players on their trade block the following day, and sends that player back to you for more negative value. This creates a bit of a racket.
This seems to be a broader feature of the trading block. I tried to see how far I could take it, and the answer seems to be...pretty far. See Jose Quintana here, who isn't free, but discounted while on the trading block
I'm on Challenge Mode, and I haven't tinkered around with any trade settings, and while I support the intuition that players on the trade block have a discount attached, maybe there should be a floor where the AI says "we'd be better off releasing this guy".