Quick question: I just accepted a trade offer for one of my spare MLB outfielders. I want to rest him until the trade goes through so he doesn't get hurt (to be fair to my trade partner), so what I want to do is designate him for assignment so I can call up a replacement from AAA and not be one bat short on the bench while waiting for the trade to go through.

Does this work? The traded player still has options left, so he would not need to clear waivers. I just don't want to risk doing anything that would screw up the trade because I need the cash it is bringing in.

You don't need to designate him if he still has options, you can just demote him; however, it may cause his ratings to drop as some players suffer a drop when demoted, so probably not a cool thing to do if you just traded him. I'd just let it be, it's only 3 games til the trade goes through.

That makes sense, thanks.

Designate without waivers. He can sit in limbo for 10 real life days and it opens a roster spot.

The point of designating is that it gets him off the ML roster and the 40-man right away, if you need the roster spot. I would think you should be okay to designate him after the trade is accepted, considering that the usual way to do it is to designate him and then offer him around anyway.