There's a big red 5 next to Rule 5 players from the day the world roles over until the day of the Rule 5 draft. Since most worlds take at least 1-3 weeks to fill up, plus a week of preseason HBD before the draft, that give everyone at least 2-4 weeks to decide which players they don't want to risk losing to Rule 5 and a full week to get it done.
If the GM in question did ANYTHING during preseason, and he's played at least one other season, I can't see any reason to give back a Rule 5 player.
In part because it sets off a domino effect. You would have gotten the next player on your list. So if you give a player back, you should be able to claim any other player picked in Rule 5 and ask that GM to give him to you. And then that GM should be able to claim the player he didn't get from someone else. Until the end of the line, That could be a dozen trades. Doable, but a pain in the ***. All because someone didn't take 5 min to put the 1 or 2 player worth worrying about on their 40 man.
Sometimes tough love is the best way to teach.