I don't think we need to make them pay anything when they sign the guy, but you are right, we definetely need a restriction to avoid day-traders. So I'm writing down the suggestion on my list like this:
Free Agency
Players fired or players from deactivated teams should become free-agents, and we would have auctions on their salary. We would need a restriction like the team that acquires a free agent cannot not sell or fire the player for an entire season, or if they want to do it before they have to buy him out (paying for him the rest of a season's worth of salary).