A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
deu 15:1THE SEVENTH YEAR, A YEAR OF RELEASE FOR THE POOR. (Deu 15:1-11)
At the end of every seven years--during the last of the seven, that is, the sabbatical year (Exo 21:2; Exo 23:11; Lev 25:4; Jer 34:14).
deu 15:2Every creditor that lendeth ought unto his neighbour shall release it--not by an absolute discharge of the debt, but by passing over that year without exacting payment. The relief was temporary and peculiar to that year during which there was a total suspension of agricultural labor.
he shall not exact it . . . of his brother--that is, an Israelite, so called in opposition to a stranger or foreigner.
because it is called the Lord's release--The reason for acquitting a debtor at that particular period proceeded from obedience to the command, and a regard for the honor, of God; an acknowledgment of holding their property of Him, and gratitude for His kindness.
deu 15:3Of a foreigner thou mayest exact it again--Admission to all the religious privileges of the Israelites was freely granted to heathen proselytes, though this spiritual incorporation did not always imply an equal participation of civil rights and privileges (Lev 25:44; Jer 34:14; compare Ch1 22:2; Ch2 2:17).
deu 15:4Save when there shall be no poor man among you--Apparently a qualifying clause added to limit the application of the foregoing statement [Deu 15:3]; so that "the brother" to be released pointed to a poor borrower, whereas it is implied that if he were rich, the restoration of the loan might be demanded even during that year. But the words may properly be rendered (as on the Margin) to the end, in order that there may be no poor among you--that is, that none be reduced to inconvenient straits and poverty by unseasonable exaction of debts at a time when there was no labor and no produce, and that all may enjoy comfort and prosperity, which will be the case through the special blessing of God on the land, provided they are obedient.
deu 15:7If there be among you a poor man . . . thou shalt not harden thine heart--Lest the foregoing law should prevent the Israelites lending to the poor, Moses here admonishes them against so mean and selfish a spirit and exhorts them to give in a liberal spirit of charity and kindness, which will secure the divine blessing (Rom 12:8; Co2 9:7).
deu 15:11For the poor shall never cease out of the land--Although every Israelite on the conquest of Canaan became the owner of property, yet in the providence of God who foresaw the event, it was permitted, partly as a punishment of disobedience and partly for the exercise of benevolent and charitable feelings, that "the poor should never cease out of the land."
deu 15:12HEBREW SERVANTS' FREEDOM. (Deu 15:12-19)
if thy brother, an Hebrew man, or an Hebrew woman, be sold unto thee--The last extremity of an insolvent debtor, when his house or land was not sufficient to cancel his debt, was to be sold as a slave with his family (Lev 25:39; Kg2 4:1; Neh 5:1-13; Job 24:9; Mat 18:25). The term of servitude could not last beyond six years. They obtained their freedom either after six years from the time of their sale or before the end of the seventh year. At the year of jubilee, such slaves were emancipated even if their six years of service were not completed [see on Lev 25:39].
deu 15:13thou 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.
deu 15:16if he say unto thee, I will not go away from thee--If they declined to avail themselves of the privilege of release and chose to remain with their master, then by a peculiar form of ceremony they became a party to the transaction, voluntarily sold themselves to their employer, and continued in his service till death.
deu 15:18he hath been worth a double hired servant to thee--that is, he is entitled to double wages because his service was more advantageous to you, being both without wages and for a length of time, whereas hired servants were engaged yearly (Lev 25:53), or at most for three years (Isa 16:14).
deu 15:19All the firstling males of thy herd and of thy flock thou shalt sanctify unto the Lord thy God--[See on Exo 13:2]; see Exo 22:30).
thou shalt do not work with the firstling of thy bullock--that is, the second firstlings (see Deu 12:17-18; Deu 14:23).