Russia interferes on Trump's behalf. Trump campaign wins, promotes policies that are beneficial to Russia (slow walking support for Ukraine, attacking NATO, generally gutting the State Department, and a plethora of other moves that cripple US soft power, edit: Oh, and how could I forget sanctions relief and readmission to G8?), and Putin approves a real estate project that benefits Trump financially. It's telling that Trump is the first president in history not to divest himself of his global financial stakes, which represent a massive conflict of interest.

Pretty basic garden variety bribery.

I agree it reads like a spy novel.

Is there any reason to believe the project would not have been approved without the alleged favors?

Does any project of that size get approved in Moscow without favors?