While on the "Strictly Business" quest, where you enslave people from around the Wasteland, Susan Lancaster will sometimes disappear from Tenpenny Tower. After hitting her with the Mesmetron and being caught, reload a saved game and try again. She may be nowhere to be found. The quest marker on the map will state you have to go to Paradise Falls. After picking Eulogy Jones' pocket for the "Box Key", gain access to the slave pens. Susan Lancaster can be found inside the adult slave barracks without a collar. Zap her with the Mesmetron and collect the payment.