At the end of each season, you will likely have players whose contracts have expired. These players will be transparent on the field and have a special contract icon when you enter the “Squad” screen. This indicates that they won’t be able to play for you or train until you’ve renewed their contract within the given time limit.
These stand-ins cannot be sold on the market or have their contract last longer until the end of the season. However, you can sack them for cash right away. In case you renew the contracts of your players who had them expire, you still get to keep the free players.
Another related word of advice is that you pay attention to the age of the players you want to buy on auctions. If the player is older than 30, there’s a chance that he might be available for only one season after you buy him. Take into consideration whether you’re fine with that if it’s still a good player or you need a temporary solution. If you’re looking for long-term team members, then the younger, the better!