The whole team in Dortmund is based on selling players to other teams. As you already said, they are not a team with strong financial power like Bayern Munich. Dortmund's first priority is not even winning the Bundesliga title but they want to sell players and earn money and that's how everything works in Dortmund.
The interesting thing about Dortmund is they even have more fans to watch games in the stadium which can show they have much more potential than other teams in Germany.
What Leverkusen did last season was really great and they went undefeated and secured the title so last season was very easy for them but the reality is that they will have to struggle a lot next season and whether they can win the Bundesliga title next season at all. There are many doubts. Bayern Munich and Dortmund could be the biggest obstacles for Leverkusen next season. These two teams are well aware of Leverkusen's tactics from last season and have spent a lot of time working on how to stop Leverkusen so it will be challenging for this team to improve on next season. Since this team has done well and has confidence in its own team, Leverkusen will not have much of a point gap with Bayern Munich and Dortmund.