If you demote a sept. call-up making league minimum will he return to a minor league salary, and will he take a demotion hit after rollover?

Yes and No.

good, thanks

What about players that were called up before roster expansions, but are making league min.?

As long as you demote them after the playoffs have started and before rollover, they will make MiLB salary next season and will NOT take a demotion hit.

Thanks for the input, I got nailed on a couple of callups. Didn't realize I needed to demote them after the playoffs started, now I'm paying ML min salaries for about 3 AAA guys.

You can actually do it in the playoffs if you're still in them. I demoted one guy with (up to) 2 games in the WS and when the world rolled, he was a Minor Leaguer, renewed at AAA salary.

One word of caution: If the guy is out of options you'll need to waive him first. If you don't pass him through waivers he'll be on your ML roster next season and making ML salary. Of course, if he's claimed off waivers then you either lose him (if waivers are nonrevokable) or he doesn't clear waivers (if waivers are revokable) and again he's on your ML roster.