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.