I think if the right offer comes, they'll let him leave.
I think so too, but it will have to be a really high offer.
From what I understand, Wolfsburg are in financially very healthy position. Only a year ago they won the Bundesliga and their intentions are to be a top team, at least in domestic level, instead of being one season wonders. Selling Dzeko will be the end of whichever dream it is that they have.
They don't play in Europe next year so improving last year's team will give them a shot for the Bundesliga because they already had a pretty solid team (that wasn't good enough for 2 strong competitions).
They are surely aware that they can't keep Dzeko for too long and sooner or later they will have to sell him. But they won't sell him until he helps them in their project or until an incredibly high offer comes. 30m eur is a lot of money indeed, but it won't seriously help Wolfsburg at this moment.