Or HaChaim on Deuteronomy 15:12/אור החיים על דברים ט״ו:י״ב

1 ועבדך, "and he will serve you, etc." The letter ו at the beginning of this word connects it to the previous commandment. This is analogous to what our sages have taught about the word לך in the sequence כי ימכר לך, that a Jewish court is not allowed to "sell" the labour of a Jew except to another Jew. The new addition to this restriction of the court's authority is that the length of time for which the labour of such a Jew may be sold is restricted to six years.

2ובשנה השביעית "and in the seventh year, etc." The words "he will leave" are noticeably missing here (compare Exodus 21,2). The Torah's intention here is that on occasion such a servant will also have to work during the seventh year, such as if he escaped during this period. Even if the original six years may have expired before he was recaptured he has to complete six years of service. This is the ruling of Maimonides in chapter two of his treatise Hilchot Avadim. The Talmud Kidushin 16 derives this from the words "six years he shall serve" in Exodus 21,2. Perhaps we could derive from that latter verse that in the event the servant falls sick for four years he remains obligated to complete six years of labour. The extra letter ו in the expression ובשנה השביעית teach that if the servant escaped he has to make up the time even if it amounts to a whole year.