John Wesley on Deuteronomy 23:15

The servant - Of such as belonged to the Canaanites, or other neighbouring nations, because if he had lived in remote countries, it is not probable that he would flee so far to avoid his master, or that his master would follow him so far to recover him. For the Canaanites this sentence was most just, because both they and theirs were all forfeited to God and Israel, and whatsoever they enjoyed was by special indulgence. And for the other neighbours it may seem just also, because both masters and servants of these and other nations are unquestionably at the disposal of the Lord their maker and sovereign ruler. Understand it likewise of such as upon enquiry appear to have been unjustly oppressed by their masters. Now it is not strange if the great God, who hates all tyranny, and styles himself the refuge of the oppressed doth interpose his authority to rescue such persons from their cruel masters.