Sforno on Leviticus 25:48/ספורנו על ויקרא כ״ה:מ״ח

1אחרי נמכר גאולה תהיה לו; our sages (Kidushin 20) have already explained that you are not to say that seeing that this Jew has sold himself and has even become a priest to a pagan cult “I will kick away this stone which has already fallen,“ i.e. not make an effort to buy his freedom. This is why the Torah had to write: “even after he has sold himself in a most undignified manner, he is entitled to be redeemed.”