when creating a floating field within a text (label) based on a value from another field in the form you must connect them with IDs.
Once you have create the floating field, select the floating field in your hierarchy and go in the XML Source of the form.
Look at the properties of the field, there should be an ID property. Take that ID value and copy the property name and value
go to your drop down list, select it and go in the XML source, and paste the ID property and value in the drop down list tag.
By default, LiveCycle will modify the property value to be a copy of the floating field, when you change the value in the drop down list, it will change the value in the floating field as well..
Hope this help!