Slave Planet by Laurence M. Janifer

Fruyling's World... rich in the metals that kept the Terran Confederation going—one vital link in a galaxy-wide civilization. But the men of Fruyling's World lived on borrowed time, knowing that slavery was outlawed throughout the Confederation—and that only the slave labor of the reptilian natives could produce the precious metals the Confederation needed!As the first hints of the truth about Fruyling's World emerge, the tension becomes unbearable—to be resolved only in the shattering climax of this fast-paced, thought-provoking story. - Summary by Book Intro