Yes, they have not announced it by themselves: They instead went to the same person they always do to make statements to the supporter base wihtout doing it officially: Freddie Röckenhaus.
https://www.sueddeutsche.de/sport/borussia-dortmund-transfer-jadon-sancho-1.4990356
(There is unfortunately no English version of that)
This article was published on August 6th, states very clearly both the 120 Mil. € figure and the deadline. The most interesting part about the article is not that, though, but the statement that both the figure and the deadline have been clearly communicated and accepted between club, the player and his agents. This points towards a confimation of a suspicion I have shared with many other supporters the whole Summer: the existence of a Gentlemans agreement between club and player. A set figure, that would allow Sancho to leave the club. This is nothing else but an unofficial release clause and just like with release clauses it is absolutelly common practice to attach a final date of activation to it.
In the end people will believe what they want to believe. Many people on here think that Dortmunds behaviour has been negotiations tactics and their primary goal is to sell Sancho this window. I believe that the unusually open nature with how they made the requirements known under which they would be entertaining the thought of selling him was them holding their end of the beforementioned bargain.