Well, there is no different transfer rule in their case. You are free to sell him. You are not allowed to fire him. That's different.
At least, buying him before the play-off helped you win your D1 as a #3 seed. The investment was worth it. Now you can use him for 2 more games on Tuesday and next Saturday while he is on sale for $1 if you absolutely want to get rid of him.
I see. I dont doubt it. It was my mistake. Its not just about him. I bought extra player already bcose i thought i can fire him. It is transferlist section in Game manual where do you read about releasing players conditions. Expect releasing NT players. I hope you can see it. I dont blame BB. I suggest BB to put one sentence to realeasing terms part of manual.