Can't you kind of already do this? Move resources to a border territory. Move units out, leaving resources. Trading partner moves in, thus taking control of resources.
OMG ... you can totally do this, and in one turn. Attack in two locations, one with the resources you want (which you ask them to throw the battle to you), and one bringing along the resources you are getting rid of (in which you throw that battle to them). You also both get a star out of it.