Slave out likely refers to the archaic practice of forcing criminals, prisoners of war, or victims of kidnapping into slavery. From the transitive verb definition of slave:
transitive verb
1 archaic : enslave
merriam-webster.com
The dialogue of Desert World Allegiances, by Lena Gala, suggests criminals sentenced to slavery:
“That we are,” Naite agreed. “And we need to make changes, but there
isn't a better solution. And I'm saying that as someone who has both
survived a long slavery sentence and is facing another."
"You ... What?" Shan asked, his stomach souring at Naite's casual
attitude toward slavery after all these horrors.
“Why would you be slaved out?” Temar sat up, his face alarmed.
“I'm likely to be slaved out to you, Temar. I don't have any training, and I definitely don't have money for fines, ... It's
that or hand over half my pay for the rest of my life, and trust me,
I'd rather be slaved out for a time and have it done and over with.”
Emphasis mine
The dialogue of The Doctrine of Presence, by Benjamin Vance, suggest prisoners of war pressed into slavery:
"...I don't know about combat, but if that happens I'll keep my head
down and let you guys take care of it.”
“What if we can't take care of it? What happens to you?”
“I get killed too, or slaved out, who the shit knows in this world,
from one minute to the next with these crazy Islamic fascists loose?"
Emphasis mine
The description of Native American Adoption, Captivity, and Slavery in Changing Contexts, by Max Carocci, et al, suggests kidnapping victims forced into slavery:
Also, the most vulnerable populations such as the mission Indians of
Spanish Florida had been slaved out. Slave catchers were simply
running out of people to enslave.
Emphasis mine
