The money that raised from selling haaland and bellingham being used to paid its expenses. The profit can be considered as good enough but it's not as big as you said. It's been reported by dortmund's financial report.
Dortmund is trying to hunt another young talented player to be developed again. I think that you will be seeing some names to join in the dortmund soon. Some names being targeted by the club this time.
In Europe, Dortmund are like Porto and Sevilla which have very good scout teams so that they can find many youth talented players around the world especially from Latin American nations and African nations to buy when those youth players are very cheap. Then they grow those players up and turn them to be star players and sell those new rising star players with expensive prices.
In the last twenty years, Dortmund are very successful with this strategy and they maintain it with acceptable good results in Bundesliga and Champions League. Obviously with this strategy, they are unable to win Bundesliga too often and especially unable to win Champions League. Anyway, winning big titles are not their priorities.