Facesitting challenge

Is this slave worth to become a human chair for these beautiful mistresses?
Princess Nikki has an idea to find out! They are testing his limits . They sit on his face 1
by 1 and stay there as long as they wish.. If he moves out, he fails.. But after
each failure he gets a few faceslaps as a punishment. But since these slaps
help him becoming a better chair, he has to compensate it with money..