Jamieson, Fausset and Brown on Deuteronomy 15:13-15

13-15. thou shalt not let him go away empty--A seasonable and wise provision for enabling a poor unfortunate to regain his original status in society, and the motive urged for his kindness and humanity to the Hebrew slave was the remembrance that the whole nation was once a degraded and persecuted band of helots in Egypt. Thus, kindness towards their slaves, unparalleled elsewhere in those days, was inculcated by the Mosaic law; and in all their conduct towards persons in that reduced condition, leniency and gentleness were enforced by an appeal which no Israelite could resist.