Dortmund is a team that very often sells the best players they have printed and almost every season is like that. I think it will not be difficult for Dortmund to find a replacement for Bellingham which they sold to Real Madrid, because selling the best players is like a business for Dortmund so I will never be surprised when in the future Dortmund sells the best players again after they have trained them with pretty good. Likewise with Bellingham which they have sold to Real Madrid.
It is Dortmund tradition and it is their strategy which was seriously applied after their club got a crisis and they carried
about 100 million euros of debt in 2005. It is because they overpaid for player transfers and salary as well as some other costs after they won Champions League in 1996 - 1997 season.
After luckily solved that crisis, Dortmund changed their strategy and focus more on finding youth players around the world, give them chances to grow up and sell them to help the club finance. They also don't agree to increase salary of their star players too much because they do know how it will be harmful for the club finance in long term.
They had a Champions League runner up in 2012 - 2013 season but about 10 years later, they did not fall into another financial crisis that proves their strategy works fine.