Sforno on Leviticus 25:50/ספורנו על ויקרא כ״ה:נ׳

1וחשב עם קונהו, even though the pagan who purchased him is legally under your authority so that you could force him to release this Jew for less than he was worth, and moreover the sale was not legal in terms of our halachah, you must compensate him fairly.