Yes, you should be able to put a guy on the trade block and then remove him whenever you want to, whether it is seconds later or days/weeks later. Unless I am misunderstanding or something new creeped into this area of the game in v4.1 then you should always be able to remove players from the trade block.
Just in case this is related:
1. Are you speaking of an online league?
2. You've double checked that the team is human controlled?
Note that leaving the player on the block does not really matter, since you would have to agree to a trade anyway, but I guess it looks good if that area of the transactions screen is neater

.