It means Dortmund are growing up sustainably and I believe they will continue with this development strategy than taking more risk to buy star players and pay them higher salary. A healthy finance from sustainable strategy is key of a club future and Dortmund are doing it well.
It's clearly regretful to revert time and look at star players they sold recent years but you can not achieve all things in life, only one or another.
That's not how it works, teams spend more on transfers than they get, because they also have other income like tv rights and merchandise and tickets etc etc. It means that they should be able to actually make some profit from other stuff that they put on top of their sales and that's what they spend on transfers.
Dortmund isn't like that, their income from transfers is higher than their spending and that makes no sense to me, look at how many amazing players they had in their history, many of them gone just recently, hell Jude himself is just 100 million, and yet when you look at what they did with that money, it is nowhere to be found. What are they doing with all of that money if they are not spending that money on buying up players?