Positive commandment · Mitzvat Aseh #114 of 613
The commandment that one who appraises a man give the value delineated in the Torah
להיות מעריך אדם נותן דמים הקצובין בפרשה
The verse · Vayikra 27:2
דַּבֵּר אֶל־בְּנֵי יִשְׂרָאֵל וְאָמַרְתָּ אֲלֵהֶם אִישׁ כִּי יַפְלִא נֶדֶר בְּעֶרְכְּךָ נְפָשֹׁת לַה׳
“Speak to the children of Yisra᾽el, and say to them, If a man make a singular vow, to give to the Lord the estimated value of persons,”
The Rambam’s words
For a person who makes an endowment valuation to give the specific amount of money stated in the [Torah] portion, as [Leviticus 27:2] states: "When a person expresses a vow...."
להיות מעריך אדם נותן דמים הקצובין בפרשה, שנאמר: "אִישׁ כִּי יַפְלִא נֶדֶר" (ויקרא כז, ב).
Why · Sefer HaChinuch #350
The commandment that one who appraises a man give the value delineated in the Torah
“It is from the roots of the commandment [that it is] since a person only belongs to the higher [realm] through speech, and this is the entire esteemed part in him.”
Who & when
“[applies] immediately.”
Where in the Mishneh Torah
Hilchot Appraisals and Devoted Property הלכות ערכים וחרמין