I think Agent was referring to the case where someone had invested in the same GB of a (later) inactive player as you did, and finally decides to level it to get some of his investment back. In that case only would you get any fps back, and only those fps in a reward slot you happened to be in when it was leveled.
You don't get anything back if the player ends the city or if Inno culls it. Once you put fps into a GB, they're gone. You only get some fps back if the GB gets leveled and you are locked into a reward spot.