A woman with a male slave III

My story is pure fiction.
English is not my native language, so please excuse any language and grammatical errors.

Daniella's niece Sophie and her black friend Bianca continue their play and teasing with Daniella's slave. They put him through a lot of humiliation and they have a lot of fun with the slave. He become used sexual by the two young women, while his own sexual need is locked in a chastity tube.
A last Daniella return from her holiday, but that doesn't mean, it will be easier for the slave.

Extract:
Sophie enjoyed the ride. She sat comfortable in the saddle and her body got the nice movements from the slave's efforts. This was fun.
But the slave was on hard work. Every times, he pulled the holders forward and backward he had to raising and lowering the rider of his back. Sophie found it funny to pull in the bridle which forced the slave's head backward and opened his mouth in a grimace. And that was not his only concerns.
Behind him stood Bianca with her whip. Every times she found he was lacking the speed she crack the whip at his naked buttocks.
After five minutes Sophie ordered the slave to stop. The slave was dripping of sweat. Sophie rose of the saddle. The two young women sipped of their drinks while they chattered and laughed. The slave remained in the machine
He gasped at the air. His ass was burning and the bridle was very unpleasant.
And as he feared, Bianca soon turned her attention at him.
She too wanted a ride.
She stepped up in the stirrup and swung her leg over before she sat down in the saddle. Both women were tall and voluptuous built, so the slave had plenty to carry, when they entered the saddle.
Sophie took her place behind the slave. She had the whip ready.
"Are you ready to begin, dear?" Sophie raised her arm with the whip.
"You bet. Send him off." The black woman sat comfortable in the saddle holding the bridle and with a big smile at her face.
"Giddy up then." Sophie yelled at the slave while she cracked the whip down at his buttocks.