Add an event to the BuildingsPage control, that the City control handles. When the BuildingsPage is clicked, it sets a property to show which building it is, and raises the event.
The City Control handles the event, retrieves the building via the property, and closes the BuildingsPage control.
It's the same process as here:
Transferring information between two forms, Part 2: Child to Parent[
^] but using controls instead of forms. City is the Parent, BuildingsPage is the Child.