Clubs just don't "stump up". You see that at every level where they often go through multiple rounds of negotations with club and then player's management.
We just don't notice it as much when it's not our player.
Even when a bid hasn't been accepted yet, managers and agents will be in discussions with potential suitors so the personal terms will be ready to sign.
Also don't forget, that most countries don't have the insanity of a league cup at the start of pre-season, and so conduct business a little bit later.
And finally, of course, MIller gets the final say on where he goes, so even if we accept a bid from a club, it doesn't mean they will get him.
Given his history of making good career choices so far, I'd be surprised if his next step isn't also ideal for him.