I found this somewhere, on some forum:
Realease Player to Free:
1. Go to the 4th option in the transfer menu - chose the team of the player you want to make free transfer. Then chose the player. Now chose the SAME team. Then chose the LAST player of that club list: Result, the player you have chosen is now last of the list of the same club. Exit that option.
2. Go to the 3rd option in the transfer menu - chose the same team. Select the player.Chose the SAME team again. Then put him on one of the free spaces at the bottom of the list - thats a free player now.
If all of your freelancers are already on PES/WE United, discharge them from there. Just use the second step above.