I believe there are a couple of issues with the Tribute system.   First,  If I demand tribute and the opposing faction has negative revenue I am actually paying a fee according to my economy screen as my income is reduced by 10% of their negative revenue.   It seems if they have no revenue my share should be 0 rather than an actual negative amount.

Also, when opposing factions offer me  tribute and give me gildar up front I end up paying them a tribute rather than receiving it.