Yeah, I had this too. The "tribute" treaty offer is apparently you give him 10% of your income. I was confused because he was weaker than me and offered money to get this treaty, and accepted it. Not like it really mattered, since I was overflowing in gold, but it was still kind of odd. Why would you offer gold, to get gold? Makes no sense to me.
Edit: Here's a SS of what we mean: