There have been plenty of rumors, and of course you won't see any "proof". Try mailing Bartomeu and ask him if he wanna share something..
Its almost certain that we have offered more than Dortmund if we have actually bid for him (which I think is highly unlikely). Both Rennes and Dembele will get more money, but Rennes will instead accept a much lower fee from Dortmund and Dembele will accept a much lower wage.
I think its very unrealistic.