Yeah, they are losing players, because they are not huge enough as a club. They can't pay top wages. Not yet. But if they have the option to say no, they WILL say no, i can assure you. Before they lose Reus next summer due to his release clause, they will try everything they can to extend his contract. But the chances are very little, because Reus is one of the best players in the world and every team will offer him crazy money.
btw. Hummels doesn't even want to leave. He said that a thousand times, that's why he has extended his contract without a release-clause. He said for example "for me personally it is more worth to win a title with a club like BVB, than with a club like Bayern, Barca, United etc. where it is nothing special and where you are just 1 star of many". As i said, the chances for United to sign him in this summer are less than 1%. The player doesn't want to leave, and even if so, BVB wouldn't sell. And no, im not BVB fan, im Bayern fan. But i just know that it is almost impossible to get Hummels in this summer.