Princess Serena did not like how lazy her slave was. She had to do something otherwise she would have to send him away. She made him kneel down in front of her and she took his money away. She humiliated him using her shoes by making him lick the dirty soles of her shoes as well as lick her feet. She then told him the money was to pay her for licking her. In essence, he was paying her to be humiliated.