Kiddushin 8A

אף שוה כסף נמי דקיץ.

so too, an item worth money must be set, i.e., it must have a clearly defined value.

אמר רב יוסף: מנא אמינא לה, דתניא: ״מכסף מקנתו״⁠ – בכסף הוא נקנה, ואינו נקנה בתבואה וכלים.

Rav Yosef said: From where do I say this opinion? As it is taught in a baraita with regard to redeeming a Hebrew slave: “He should give back the price of his redemption out of the money that he was bought for” (Leviticus 25:51), which indicates: He is acquired specifically through money and he is not acquired through grain or vessels.

האי תבואה וכלים היכי דמי? אילימא דלא מקנו בהו כלל, ״ישיב גאלתו״ אמר רחמנא ⁠– לרבות שוה כסף ככסף;

Rav Yosef explains: What are the circumstances of these grains and vessels? If we say that a Hebrew slave cannot be acquired through them at all, as money alone may be used, God states: “He should give back the price of his redemption,” which serves to include all modes of repayment as valid equivalents of money, i.e., an item worth money is the same as money. There is no requirement to use money in particular; it is also possible to use items with monetary value.

ואי דלית בהו שוה פרוטה, מאי איריא תבואה וכלים, אפילו כסף נמי! אלא לאו דאית בהו שוה פרוטה, וכיון דלא קייצי לא?

And if you say that they do not have the value of one peruta, why mention specifically grain and vessels? Even money that is not worth one peruta cannot be used for acquiring a slave. Rather, is it not speaking here about a case where the grain and vessels do have the value of one peruta, but since they lack a set value, no, a Hebrew slave cannot be redeemed with them? The comparison with money teaches that a Hebrew slave can be redeemed only with an item that has a clearly defined value, like money.