Jupiter is to be used as an advertising billboard), to be seen by passing spacecraft. The main Earth negotiator reveals to his colleagues that he has outsmarted the aliens, who clearly are not experienced hagglers, having neglected the other Jovian planets.
To advertise their product, Jupiter will serve as a billboard for passing spacecraft.  The Earth negotiator successfully outsmarted the alien negotiators by exploiting their lack of knowledge about other Jovian planets during negotiations.