How it is working :

If you have a player with 39-59-79-99...% quality, he is in the nego list
If you have an offer for him, you can't put him for training. Also during the offer, he won't get the gift skill (4% skills) from friends
That's because from a gift or training he can go from 59% to 60% and get out from nego list.
If he has 99% condition, he wont earn any condition of 6% resting.
99% it's the max.

But you can use him in a game, he won't earn any skills, gonna loose condition and get it back from 6% resting.