Ademola Lookman's big problem is his age - he is not old, but he is not young either. Atalanta is asking 50 million for his card, but whoever buys him knows very well that he will make a capital loss. It will no longer be possible to sell him for more, so those who courted him were hoping for a discount from Atalanta, a discount that never came.
I know that Inter were very close to buying him but only went as far as 45million, but that wasn't enough to convince Percassi.
Now Atalanta risk losing him for free if they don't find a buyer.
Lookman could be priced less than 50m if the buyer was non italian club. However, he always gets false hope from non italian club. Meanwhile, italian club is also very reluctant in paying 50m for him. Like you said, it most likely he will leave for free. Atalanta can't offer his potential anymore as he's already 27 years.
Things worsen when he was behaving like that. Impossible for Atalanta to recoup at least 30m from him next season.
I consider it's Atalanta's mistake.