Paying €40 million for Lookman wouldn't be too difficult for Inter. I'm sure they're currently conducting a thorough research to determine if he's the right player. If the research is successful, Inter will sign him. Last year, they lost in both the Champions League and Italian league finals, and they'll be looking to win a trophy this year.
40m was Inter's offer for Lookman, not Atalanta. Atalanta wanted 55m for Lookman. It's because they're reluctant in selling their player to their rival. In fact, Inter can't afford Atalanta's asking price. The better inter to find Lookman alternative that can fit their budged. 
In terms of individual quality, I think Lookman has proven himself, but Atalanta is currently trying to hold on to their star player. There is no replacement for Lookman in the squad for next season, so Atalanta is unlikely to sell him. This is their mainstay player, and selling him would be very risky for the team's performance.
Selling Lookman will certainly impact Atalanta's performance, and finding a player like Lookman won't be easy. However, it would be better if Inter Milan found another player with similar abilities but at a price Inter Milan can afford. However, it seems Atalanta lacks the ambition to sell him to a Serie A club, but if he does go to another club, there's certainly potential for Lookman to be released. But of course, we'll just have to wait and see what happens, because the most important thing, in my opinion, is to always be lucky when betting.
However, the discussion and talk about Lookman are certainly never-ending. Lookman certainly seems to want to leave Atalanta, but any team interested in him would need strong funding, and Atalanta would certainly prioritize a non-Serie A team. We'll just have to see whether Inter can sign Lookman. Many clubs, including Arsenal, Atletico Madrid, and others, are interested in him. The one who gets him the quickest will undoubtedly get him. It's exciting to watch him now, and the outcome will undoubtedly come eventually.