Efron offered it for nothing, then named his price. Avraham weighed out every shekel in front of the city gate.
וַיִּשְׁמַע אַבְרָהָם אֶל עֶפְרוֹן וַיִּשְׁקֹל אַבְרָהָם לְעֶפְרֹן אֶת הַכֶּסֶף אֲשֶׁר דִּבֶּר בְּאׇזְנֵי בְנֵי חֵת אַרְבַּע מֵאוֹת שֶׁקֶל כֶּסֶף עֹבֵר לַסֹּחֵר׃
בראשית כג:טז
"Avraham listened to Efron, and Avraham weighed out for Efron the silver he had named in the hearing of the children of Ches: four hundred shekels of silver, current with the merchant."
Bereishis 23:16
Only once the field was his did he bury her.
וְאַחֲרֵי כֵן קָבַר אַבְרָהָם אֶת שָׂרָה אִשְׁתּוֹ אֶל מְעָרַת שְׂדֵה הַמַּכְפֵּלָה עַל פְּנֵי מַמְרֵא הִוא חֶבְרוֹן בְּאֶרֶץ כְּנָעַן׃
בראשית כג:יט
"And after this, Avraham buried Sarah his wife in the cave of the field of Machpelah, facing Mamre, which is Chevron, in the land of Canaan."
Bereishis 23:19
Years later, both his sons came back to the same cave. Rashi says Yishmael let Yitzchak go first.
וַיִּקְבְּרוּ אֹתוֹ יִצְחָק וְיִשְׁמָעֵאל בָּנָיו אֶל מְעָרַת הַמַּכְפֵּלָה אֶל שְׂדֵה עֶפְרֹן בֶּן צֹחַר הַחִתִּי אֲשֶׁר עַל פְּנֵי מַמְרֵא׃
בראשית כה:ט
"Yitzchak and Yishmael his sons buried him in the cave of Machpelah, in the field of Efron son of Tzochar the Hittite, which faces Mamre."
Bereishis 25:9