Adam Clarke on Job 31:13

The cause of my man-servant - In ancient times slaves had no action at law against their owners; they might dispose of them as they did of their cattle, or any other property. The slave might complain; and the owner might hear him if he pleased, but he was not compelled to do so. Job states that he had admitted them to all civil rights; and, far from preventing their case from being heard, he was ready to permit them to complain even against himself, if they had a cause of complaint, and to give them all the benefit of the law.