It would be quite weird to first pay 20,000 gold for her and then simply kick her out a while later, without a good reason. Maybe if the duke's family line is supposed to possess some magic power or whatever, and it turned out she doesn't have it. Otherwise they don't seem to expect anything from her, nor are they especially inspecting her. Carles was first shown sick, so it did visit my mind the duke could have bought her to be used for spare parts as well.