Mishnayos Eruchin Perek 9 Mishnah 4

If the final day of the twelve-month period arrived and the house was not redeemed, the house has become the property of the buyer in perpetuity. This is the halakha with regard to both one who buys a house in a walled city and one to whom it is given as a gift, as it is stated: “Then the house that is in the walled city shall stand in possession of the one who bought it in perpetuity” (Leviticus 25:30). At first, the buyer would conceal himself on the final day of the twelve-month period, in order to ensure that it would become his in perpetuity. Hillel instituted that the seller would place [ḥolesh] his money in the chamber of the court and that he will break the door and enter the house, and when the other individual, i.e., the buyer, will wish to do so, he may come to the chamber and take his money.

הִגִּיעַ יוֹם שְׁנֵים עָשָׂר חֹדֶשׁ וְלֹא נִגְאַל, הָיָה חָלוּט לוֹ, אֶחָד הַלּוֹקֵחַ וְאֶחָד שֶׁנִּתַּן לוֹ מַתָּנָה, שֶׁנֶּאֱמַר לַצְּמִיתוּת. בָּרִאשׁוֹנָה הָיָה נִטְמָן יוֹם שְׁנֵים עָשָׂר חֹדֶשׁ, שֶׁיְּהֵא חָלוּט לוֹ. הִתְקִין הִלֵּל הַזָּקֵן, שֶׁיְּהֵא חוֹלֵשׁ אֶת מְעוֹתָיו בַּלִּשְׁכָּה, וִיהֵא שׁוֹבֵר אֶת הַדֶּלֶת וְנִכְנָס. אֵימָתַי שֶׁיִּרְצֶה הַלָּה, יָבֹא וְיִטֹּל אֶת מְעוֹתָיו: