You are right that Lewandowski should not be forced to stay at Bayern Munich. Because if the players are not mentally happy then their performance is not good. Lewandowski should be let go. Since Bayern Munich have picked Mane as a replacement for Lewandowski, Bayern Munich will not have too many problems next season.
However, if Barcelona want to sign Lewandowski, they will have to re-fund. Since they are in financial crisis, some players may need to raise funds through sales.
Lewandowski is not mentally prepared to remain at Bayern Munich next season. It would be unfair to force him to stay since he can't imagine further positive cooperation. It is hoped that Bayern Munich will not keep him merely because they are able to do so due to his contract. Despite the fact that he wants to win the LaLiga league and possibly the Ballon d'Or award, he wants to join Barcelona so badly. However, money talks, but Barcelona's current state of finances is unbalanced, which prevents further progress.