Sforno on Leviticus 25:40/ספורנו על ויקרא כ״ה:מ׳

1כשכיר כתושב יהיה עמך. A Jewish servant whose labour has been acquired and paid for 6 years in advance by the person hiring him shall be treated as if he had been hired by the year. If someone’s labour has been contracted for by you until the advent of the next Jubilee year he is to be treated as if he had been hired with a long-term contract.