See I'm not ITK when it comes to Dortmund and their transfer strategy, but I'm fairly sure If Dortmund had decided to only sell 2 players this summer earlier, Gundogan and Hummels were going to be the selected 2 regardless of how quickly Woodward moved for Mkhitariyan. Like you said, if you look at what Watzke said, he clearly mentions that he didn't have a prior agreement with Miki that he could leave regardless of whether he signed a new contract or not. So I'd assume both Gundogan and Hummels had a verbal agreement that they could leave.
Secondly, with regards to getting the money Dortmund want, asking for around €50m or even €40m surely isn't realistic for a a player with only a year left on his contract? I mean Miki was class this season, but he's at best a €45/50m signing even with more years left on his contract. As you'd have to factor in his age as at 27, in say 3 years time he'd have little sell back value. Even if we were to compare with De Bruyne who City overpaid for, De Bruyne, who had a longer contract and had just turned 24 after having a similar season to what Miki just had (De Bruyne: 10 goals, 21 assists vs Miki: 11 goals, 20 assists in Bundesliga). £25m to £30m seems a very fair fee surely?