ive just read everyones post and it seems to me unless you get him early you will have to end up paying 100m or somethink which i think its unrealistic because we all know that hes worth at least 30-40m irl also irl dont think he is the player everyone makes out to be , hes more of player like juan mata than a messi type.
Do like someone here said, build up a reputation, keep praising him, and if Dortmund begins to struggle, he'll want to leave. You'd get him cheaper, maybe even for free if Dortmund refuses to sell him (he might not want to sign a new contract in that case). But you can't be sure this ever happens, and if it does it could take a long time...