He was a product of Bayern Munich's academy system in the first place and BvB got him for around 3 million pounds back in 2008. In that sense, i believe that Dortmund will get some satisfaction from the fee they are going to receive from Bayern for him (even though it won't be a very high fee). Plus most of the time they know how to invest their money wisely. And if they stick to their "never sell more than one first team players per season" principle, this could potentially mean bad news for Klopp or whoever else will look towards their direction for reinforcements this summer.
As for Hummels, he has been at the club for almost a decade and has played a major role in its recent success. It's bad for the fans but, at the end of the day, he is a professional footballer and Bayern Munich guarantee the best wages in the country and their most recent status says they are a "top three in the world" club right now. Hummels, 28 next season, is probably going to sign the last big contract of his career and he's about to enter the most fruitful period in a defender's playing career. All these things matter whether, we fans, like it or not.
I reckon BvB will also be ok. If they manage to fully convert Bender to a CB in a high defensive line, they won't even have to make a big investment at the back. And Tuchel has shown that he knows how to manage and protect his players through rotation.
And i don't think there's nothing Dortmund can do since investors aren't allowed to throw money at a club in Germany. They won't be able to match Bayern's wages ever and most German players (or players who made their breakthrough in the Bundesliga) will always be tempted to play for Bayern Munich and not leave the country and its culture. If Dortmund decide to play hard ball and bench first team players or send them to train with the kids, all they'll achieve will be to make top talents choose other (domestic) destinations or sign very short term contracts with them. The biggest problem is that there aren't any other German clubs who can offer the same money and wages as Bayern, so that the players will have more than one destinations to choose from.
As for the "bench him" option... Yeah, bench Germany's first choice CB in a Euro season, that will work out fine
. He would leave anyway and Dortmund would have UEFA, the German FA and most of the country against them... sounds like a plan.