Commentary for Kiddushin 40:20
שיכול והלא דין הוא ומה מוכר שדה אחוזה שיפה כחו שאם הגיע יובל ולא נגאלה חוזרת לבעליה ביובל הורע כחו שאין לוה וגואל וגואל לחצאין מקדיש שהורע כחו שאם הגיע יובל ולא נגאלה יוצאה לכהנים ביובל אינו דין שהורע כחו שאין לוה וגואל וגואל לחצאין
Because we find in the case of him who sells 'a field of possession', that [since] he has a great privilege, in that if jubilee comes and it not been redeemed, it reverts to its owners, his rights are weakened in [so far] that he cannot borrow and redeem, and redeem half; hence he who sanctifies ['a field of possession'] whose rights are impaired in that if jubilee comes and it has not been redeemed, it goes out to the priests at jubilee, [therefore] his privilege strengthened in [so far] that he may borrow and redeem, and redeem half. Hence this one too, who sells a house in a walled city, since his rights are impaired so that if a complete year elapsed and it is not redeemed, it is absolutely [sold], therefore his privilege is strengthened in that he can borrow and redeem, and redeem half.