Dude's pretty chill, considering he just lost everything he had because his slave couldn't keep her fists to herself.
My guess is he didn't have that much to begin with, and he probably saw things getting spicier in the war in the near future.
I am hoping for a plot twist where she is some sort of high ranking knight, who runs around recruiting knights in her order by being a slave, a gladiator slave who ended up getting sent to the wrong place, some sort of hyper bizarre fighter training, or a demon lord who is a hyper bottom.
Or this is like chikatoon and the slaver has employed Dr Brainwash to make all the slaves super happy and obedient. I am expecting MC to end up on the other team he was fighting, defeating that lord, then getting the noble daughter as a slave...