Positive commandment · Mitzvat Aseh #117 of 613
For a person who consecrates his field to give the fixed amount determined by the [Torah]
להיות מקדיש שדהו נותן בערך הקצוב בפרשה
The verse · Vayikra 27:16
וְאִם מִשְּׂדֵה אֲחֻזָּתוֹ יַקְדִּישׁ אִישׁ לַה׳ וְהָיָה עֶרְכְּךָ לְפִי זַרְעוֹ זֶרַע חֹמֶר שְׂעֹרִים בַּחֲמִשִּׁים שֶׁקֶל כָּסֶף
“And if a man shall dedicate to the Lord some part of a field of his possession, then the estimation shall be according to the seed required for it, a ĥomer of barley seed shall be valued at fifty shekels of silver.”
The Rambam’s words
For a person who consecrates his field to give the fixed amount determined by the [Torah], as [Leviticus 27:16] states: "And the value you attach to it shall be according to the amount of seed."
להיות מקדיש שדהו נותן בערך הקצוב בפרשה, שנאמר: "וְהָיָה עֶרְכְּךָ כְּפִי זַרְעוֹ" (ויקרא כז, טז).
Who & when
“And [it] is practiced by males and females at the time that the Jubilee is practiced.”
Where in the Mishneh Torah
Hilchot Appraisals and Devoted Property הלכות ערכים וחרמין