I thought so too, but it won't let me. There's no options for assignment in the dropdown box. It says this in the white part: Posiedon Towers may not be assigned back to the big league team since he was designated for assignment. You may trade him, release him, or assign him to a minor league club if he clears waivers and accepts the demotion.
I'm guessing he's refusing the demotion? It's the only thing I can figure. I'm planning to wait until the 10 DFA days are up and see what happens.
Obviously in retrospect I should have just cut him instead of putting him on waivers, but now that I owe him 2.3 mil either way I'd like to keep him around if possible.