I believe this is the same issue I ran into months ago: Incorrect gold amount Undercity tributes - can't add passive ranks
In typical Blizzard fashion I don’t think they actually solved the problem, but they instead simply fixed the visual of the bugged gold amount. So it likely thinks you don’t have enough gold. I fixed it by spending at least 50m in enchanting for instance, which put me in the negatives according to the bugged tribute counter - but the game accepted it.