Midrash su Levitico 27:20

5) (another answer:) Because it is written (Vayikra 25:30) "And if it (the house) is not redeemed until its fulfillment of a complete year, etc.", I might think (that this restriction applies) even to the Levites, it is, therefore, written "a perpetual redemption shall there be for the Levites." (another answer):) Because it is written (Vayikra 27:20-21) "And if he does not redeem the field, and if he sells the field to another man, it shall not be redeemed again. And the field shall be, when it goes out on the Yovel, holy to the L–rd, as a devoted field; to the Cohanim shall be his holding" — I might think that the same applies to the Levites; it is, therefore, written "a perpetual redemption shall there be for the Levites."

5) (another answer:) Because it is written (Vayikra 25:30) "And if it (the house) is not redeemed until its fulfillment of a complete year, etc.", I might think (that this restriction applies) even to the Levites, it is, therefore, written "a perpetual redemption shall there be for the Levites." (another answer):) Because it is written (Vayikra 27:20-21) "And if he does not redeem the field, and if he sells the field to another man, it shall not be redeemed again. And the field shall be, when it goes out on the Yovel, holy to the L–rd, as a devoted field; to the Cohanim shall be his holding" — I might think that the same applies to the Levites; it is, therefore, written "a perpetual redemption shall there be for the Levites."

Moses cried out: My master, You did not command me about even one of the seventy nations, only concerning Israel: When thou takest the sum of the children of Israel, Unto the children of Israel thou shalt speak, And unto the children of Israel thou shalt say, Command the children of Israel, Speak unto the children of Israel. You commanded me to do this only to the children of Israel. He replied to him: I did so because they are dearer to me than all the nations. They are My treasure, I love them and have chosen them, as it is said: And the Lord hath chosen thee to be his own treasure out of all peoples (Deut. 14:2). Observe how precious they are that they are mentioned five times in a single verse: I have given the Levites—they are given to Aaron and his sons from among the children of Israel, to do the service of the children of Israel in the Tent of Meeting, and to make atonement for the children of Israel, that there be no plague among the children of Israel, through the children of Israel coming nigh unto the Sanctuary (Num. 8:19).