Deuteronomy 23:15

To thee, from among the Gentiles. The promised land was thus declared a land of liberty, (Calmet) to encourage poor slaves to embrace the service of the true God, and to flee from the slavery of the devil, and from the society of those who adored him in their idols. The whole earth belongs to the Lord, and He was thus pleased to punish those who might claim a right to these slaves. (Haydock)

Some believe that the price was given to the owner, at the public expense. The Rabbins allow this privilege of an asylum, only to those who fled from a foreign country, or from an infidel master, to embrace the true religion. Circumcision was given to them as an inviolable mark of liberty. (Chaldean) Those who had been sold for their crimes, or for debt, by the sentence of the judge, could not claim this exemption. (Grotius, Jur. iii. 7.)

Philo (de humanit.) says, it would be unjust to give up a slave who has sought refuge with us. We ought either to reconcile him to his master, or sell him to another, and give the price to the former owner. Some translate the Hebrew in a contrary sense, "Thou shalt not shut up the slave who has fled to thee from his master," as if it were unjust to refuse to deliver him up. But the law points out some cases where it is lawful for a slave to flee away, and consequently people must be allowed to receive him. The following verse is decisive in favour of this explanation.