Haha, the post 2 above mine replied to yours giving you one source (times).
A quick google will give you plenty more.
When I google "haaland €50 mil release clause" the first link I get is the one from times, which is paywalled, but google suggest they wrote "in excess of €50m" somewhere in that article, second link is Odgen probably copying from the times, third one is a generic "Haaland joined Dortmund article" that doesn't mention specifics and the fourth one is a link to this thread.