As far as I understand, this isn't a bug. It just isnt a quantified trade. I thought it was a bug too at first! But it's just the opponent telling you "I want some wheat, what would you like for it?"
You can select the ressources you want for it and return your answer to see if he accepts.
You can also do the same and ask for one ressource without asking for anything, and they will tell you what they want for it... Sometimes!! AIs are tough when the trade comes from you.