Caledorn
Emperor
We should tell them "Ok, in the spirit of compromise you can settle at {U1 spot} to get the Ivory."
I don't like using the word "compromise" at this time. We need to be a bit obeiscant at the moment, to sweeten things as much as possible as we really need this NAP.
I suggest something along the lines of "We were not aware of the Ivory resource. We have no objections to you settling a city at {U1}, and we hope you have no objections to us settling at {X1}, as we need the marble near that spot. If you do object to us settling there feel free to let us know, and we can work out a different arrangement."