Why would you want evidence that something illegal was taking place if something illegal was taking place? Sure in the absence of evidence I do believe we cannot declare anyone guilty, but the presumption of innocence doesn't mean we can't cast suspicion.

EDIT: Responding to your answer below. You are materially misreporting the substance of the NYT article, which I find to be telling. It says that the building was sold for $8 million two years before, and that Trump paid "less than" that.