Or HaChaim on Exodus 21:2/אור החיים על שמות כ״א:ב׳

1 כי תקנה עבד עברי, "When you purchase a Jewish slave, etc." Perhaps the meaning is: "when you are about to buy a slave, buy a Jewish slave (rather than a Gentile)." You should not buy a Gentile because you know you can keep him indefinitely.

2The reason the Torah refers to the slave as עבדי instead of ישראלי, may be that the Torah is sensitive about combining the words עבד and ישראל. The Torah also wants us to know that the term עבד when used as applicable to a Jew denotes a temporary status only, seeing that all Jews are permanently G'd's servants. This is one of the reasons such a Jewish servant must leave his master in the seventh year. The Torah also hints that unless a Jew had violated the Torah's commandments he would not find himself in the position of being a slave. Kiddushin 14 in particular states that the Torah speaks of a thief who was unable to make restitution and who has been "sold" by the court in order that the proceeds of the sale be used to compensate his victim. Even in a situation discussed by the Torah in Deut. 15,12 when the subject has sold himself, he did not find himself in such a desperate situation had he not previously violated Torah law. Rabbi Ami tells us in Shabbat 55 that afflictions have always been preceded by sins committed by the individual afflicted.

3Another nuance in the words עבד עברי instead of עברי עבד, "a Jew as a slave," is a reminder to the purchaser that the individual in question was an עבד already before the present master had purchased him. The idea is that he was already guilty vis-a-vis Heaven. The court would not sell him until he had been convicted. As a result we need not have any misgivings about the term עבד עברי being used by the Torah in this instance as opposed to Deut. 15,12 where the Torah characterises the "slave" as אחיך העברי, "your brother the Jew."

4The term עבד had to be used by the Torah to justify why he has to serve his master for six years and cannot leave his employer like ordinary employees.

5שש שנים יעבוד, he shall serve for six years, etc. The Torah means that even if the slave escaped during these six years he has to complete the term remaining from the original six years. The reason the Torah does not write: "he has to serve you," (i.e. the purchaser) is to allow for the slave to complete the six years if the original master has died and he has now become the property of the son.

6ובשביעית יצא לחפשי חנם, and in the seventh year he leaves to become free without payment. The Torah means that there are occasions when said slave has to serve also in the seventh year such as when he has been sold in the middle of a calendar year. The six years are counted as commencing the day he is sold. The Torah also indicates by this verse that if the "seventh year," i.e. the שמטה occurs during the six years the slave has to serve, his term of service remains unaffected by that fact. I have found this in the commentary of Maimonides on Kiddushin chapter 1, Mishnah 2.