When you click on the building it should be the only picked building, so just set your text object to building.Name, or whatever your instance variable is. An alternative for the Selected instance variable is a Global that is set to the UID of the currently selected building. Then whenever you need to do something with that building, you can pick it by UID, with that global. Using an instance variable can be done, but you need a Pick all buildings first, so you can set the value to 0, then when you pop back to the top level you set the selected value to 1.
It would be easier if you posted a CAPX. It will save time for any further questions.