Halacha

הלכה א
הַמּוֹכֵר לַהֶקְדֵּשׁ וְאָמַר לוֹ הַגִּזְבָּר בְּכַמָּה אַתָּה מוֹכֵר חֵפֶץ זֶה וְאָמַר בַּעֲשָׂרָה אֲפִלּוּ הָיָה שָׁוֶה מֵאָה כֵּיוָן שֶׁאָמַר בַּעֲשָׂרָה אֵינוֹ [יָכוֹל] לַחְזֹר בּוֹ. שֶׁאֲמִירָה לְגָבוֹהַּ כִּמְסִירָה לְהֶדְיוֹט:
כסף משנה
1.
The following rules apply when a person sells an article to the Temple treasury. If the representative of the treasury asks him: "How much are you selling it for?" and the person says: "Ten zuz" even if it is worth 100, once he said ten he cannot retract. For making a promise to the Most High is considered equivalent to transferring the article in question to the possession of a colleague.

הלכה ב
הַגִּזְבָּר שֶׁקָּנָה לַהֶקְדֵּשׁ אוֹ שֶׁמָּכַר יָדוֹ עַל הָעֶלְיוֹנָה. כֵּיצַד. נָתַן דָּמִים שֶׁל הֶקְדֵּשׁ אַף עַל פִּי שֶׁלֹּא מָשַׁךְ הַפֵּרוֹת אִם הוּקְרוּ קָנָה כְּדִין תּוֹרָה. וְאִם הוּזְלוּ הַפֵּרוֹת חוֹזֵר שֶׁהֲרֵי לֹא מָשַׁךְ. וְלֹא יְהֵא כֹּחַ הֶדְיוֹט חָמוּר מִכֹּחַ הַהֶקְדֵּשׁ. וְכֵן אִם מָכַר חֵפֶץ שֶׁל הֶקְדֵּשׁ וּמְשָׁכוֹ הַלּוֹקֵחַ וְלֹא נָתַן דָּמִים וְהוּזַל הַחֵפֶץ קָנָה. שֶׁלֹּא יְהֵא כֹּחַ הֶדְיוֹט חָמוּר מִכֹּחַ הֶקְדֵּשׁ. וְאִם הוּקַר הַחֵפֶץ חוֹזֵר בּוֹ שֶׁהֲרֵי לֹא לָקַח הַגִּזְבָּר דָּמִים. וְהַהֶקְדֵּשׁ אֵינוֹ נִקְנֶה אֶלָּא בְּכֶסֶף שֶׁנֶּאֱמַר וְנָתַן הַכֶּסֶף וְקָם לוֹ. וְאֵין הַגִּזְבָּר חַיָּב לְקַבֵּל מִי שֶׁפָּרַע:
כסף משנה
2.
Whether a representative of the treasury buys or sells an article, he is always given the upper hand if there is a fluctuation in the article's value.
What is implied? If the representative paid with money from the Temple treasury, but did not perform meshichah on the produce, although the value of the produce increases, the Temple treasury nevertheless acquires the produce, as prescribed by Scriptural Law. If the value of the produce decreases, the Temple treasurer may retract, for he did not perform meshichah, and the legal power of an ordinary person should not be greater than that of the Temple treasury.
Similarly, if the Temple treasurer sold an article that had been consecrated, the purchaser performed meshichah, but did not pay for it, and then the value of the article decreases, the transaction is finalized. For the legal power of an ordinary person should not be greater than that of the Temple treasury. If the value of the article increases, the representative of the treasury may retract, for he did not accept payment, and consecrated articles may be acquired only through the payment of money, as implied by the verse: "And he shall give the money and it will be established as his." The representative of the treasury is not liable to receive the adjuration mi shepara.

הלכה ג
נִכְסֵי יְתוֹמִים הֲרֵי הֵן כְּהֶקְדֵּשׁ. כֵּיצַד. יְתוֹמִים שֶׁמָּכְרוּ פֵּרוֹת וְנִמְשְׁכוּ מֵהֶן הַפֵּרוֹת וַעֲדַיִן לֹא לָקְחוּ הַדָּמִים וְהוּקְרוּ הַפֵּרוֹת חוֹזְרִין בָּהֶן. שֶׁאֵין נִכְסֵי יְתוֹמִים נִקְנִין אֶלָּא בְּכֶסֶף כְּהֶקְדֵּשׁ. הוּזְלוּ הַפֵּרוֹת לֹא יִהְיֶה כֹּחַ הֶדְיוֹט יוֹתֵר חָמוּר מִכֹּחָן:
כסף משנה
3.
Property belonging to orphans is governed by the same rules as those governing the Temple treasury.
What is implied? When orphans sell produce, and meshichah is performed on the produce, but they have not yet received the money for it, they may retract from the agreement if the value of the produce increases. For like consecrated property, property belonging to orphans can be acquired only through the transfer of money.
If the value of the produce decreases the transaction should be allowed to stand. For the legal power of an ordinary person should not be greater than the legal power of orphans.

הלכה ד
וְכֵן אִם לָקְחוּ הַדָּמִים וַעֲדַיִן לֹא נִמְשְׁכוּ פֵּרוֹתֵיהֶן וְהוּקְרוּ. חוֹזְרִין כִּשְׁאָר הַהֶדְיוֹטוֹת. אֲבָל אִם הוּזְלוּ הַפֵּרוֹת וְרָצוּ הַלָּקוֹחוֹת לַחְזֹר בָּהֶן. חוֹזְרִין וּמְקַבְּלִין מִי שֶׁפָּרַע כְּדִינָם עִם שְׁאָר הָעָם. שֶׁאִם נְחַיֵּב אוֹתָם לִקַּח הַפֵּרוֹת כְּדִין תּוֹרָה הֲרֵי זוֹ רָעָה לַיְתוֹמִים. שֶׁאִם יִהְיֶה דִּינָם כָּךְ כְּשֶׁיִּצְטָרְכוּ לִמְכֹּר לֹא יִמְצְאוּ מִי יִתֵּן לָהֶם דָּמִים:
כסף משנה
4.
Similarly, if orphans received payment, but meshichah was not performed on their produce before it increased in value, they may retract as may ordinary individuals. If, however, the value of the produce decreased and the purchasers desire to retract, they may do so, but they must receive the adjuration mi shepara, as would be the law with regard to others.
They are allowed to retract because if we required them to keep the produce, as would be required by Scriptural Law, this would be a disservice to the orphans. For if this were the law, when the orphans desire to sell an object, they would never find anyone who would be willing to pay them money unless the object were handed over first.

הלכה ה
וְכֵן יְתוֹמִים שֶׁלָּקְחוּ פֵּרוֹת וּמָשְׁכוּ הַפֵּרוֹת וְלֹא נָתְנוּ הַדָּמִים וְהוּקְרוּ לֹא יִהְיֶה כֹּחַ הֶדְיוֹט גָּדוֹל מִכֹּחָן. וְאִם הוּזְלוּ הַפֵּרוֹת אֵינָן חוֹזְרִין. שֶׁזּוֹ רָעָה לָהֶם כְּשֶׁיִּצְטָרְכוּ לִקְנוֹת פֵּרוֹת לֹא יִמְצְאוּ מִי שֶׁיִּמְכֹּר לָהֶם:
כסף משנה
5.
Similar principles apply if orphans purchase produce and perform meshichah on the produce, but have not yet paid for it. If the value of the produce increases, the legal power of an ordinary person should not be greater than their legal power. Therefore, the transaction should be allowed to stand."
If the value of the produce decreases, they are not allowed to retract. For this would be a disservice to them. If they were given this privilege, should they desire to purchase produce, they would not find anyone who would sell it to them before they made payment.

הלכה ו
נָתְנוּ הַדָּמִים וְלֹא מָשְׁכוּ הַפֵּרוֹת וְהֻזְּלוּ הַפֵּרוֹת חוֹזְרִין. לֹא יִהְיֶה כֹּחַ הֶדְיוֹט גָּדוֹל מִכֹּחָן. הוּקְרוּ אִם רָצוּ הַמּוֹכְרִים לַחְזֹר בָּהֶן חוֹזְרִין וּמְקַבְּלִין מִי שֶׁפָּרַע. שֶׁאִם יִהְיֶה דִּינָם שֶׁיִּקְנוּ בִּנְתִינַת הַמָּעוֹת יֹאמַר לָהֶם הַמּוֹכֵר נִשְׂרְפוּ הַפֵּרוֹת שֶׁלְּקַחְתֶּם אוֹ אָבְדוּ בְּאֹנֶס וּכְבָר נַעֲשׂוּ בִּרְשׁוּתְכֶם מִשְּׁעַת נְתִינַת הַמָּעוֹת:
כסף משנה
6.
If the orphans paid for produce but did not perform meshichah, and then the value of the produce decreases, they may retract, for the legal power of an ordinary person should not be greater than their legal power. If the value of the produce increases, the sellers may retract if they desire and receive the adjuration mi shepara. For if the law were that the orphans acquire the produce by the payment of money, the seller would tell them that the produce was destroyed by fire or another factor beyond human control after the produce entered their property, at the time payment was made.

הלכה ז
בְּאַרְבָּעָה פְּרָקִים בַּשָּׁנָה הֶעֱמִידוּ דִּבְרֵיהֶם עַל דִּין תּוֹרָה בְּבָשָׂר מִפְּנֵי שֶׁהָעָם כֻּלָּן צְרִיכִין לְבָשָׂר. וְאֵלּוּ הֵן. עֶרֶב יוֹם טוֹב הָאַחֲרוֹן שֶׁל חַג. וְעֶרֶב יוֹם טוֹב הָרִאשׁוֹן שֶׁל פֶּסַח. וְעֶרֶב הָעֲצֶרֶת. וְעֶרֶב רֹאשׁ הַשָּׁנָה. כֵּיצַד. הָיָה לַטַּבָּח שׁוֹר אֲפִלּוּ שָׁוֶה מֵאָה דִּינָרִין וְלָקַח דִּינָר אֶחָד מִן הַלּוֹקֵחַ כְּדֵי לִתֵּן לוֹ בָּשָׂר כְּשֶׁיִּשְׁחֹט וְלֹא נִתְקַבְּצוּ לוֹ כָּל דְּמֵי הַשּׁוֹר אֵינוֹ יָכוֹל לַחְזֹר בּוֹ. אֶלָּא מַשְׁחִיטִין אֶת הַטַּבָּח בַּעַל כָּרְחוֹ. וְכוֹפִין אוֹתוֹ לִשְׁחֹט וְלִתֵּן הַבָּשָׂר לַלּוֹקֵחַ. לְפִיכָךְ אִם מֵת הַשּׁוֹר מֵת לַלּוֹקֵחַ:
כסף משנה
7.
On four occasions during the year, our Sages restricted their enactments and applied Scriptural Law with regard to the purchase of meat, for on these days, all people need meat. The times are the day before Shemini Atzeret, the day before the first day of Pesach, the day before Shavuot and the day before Rosh HaShanah.
What is implied? If a butcher had a steer that was worth even 100 dinarim, and he took one dinar from the purchaser in exchange for meat that he would slaughter, he cannot retract, even if enough money is not collected to pay for the entire value of the steer. Instead, the butcher is compelled to slaughter against his will. He is forcibly made to slaughter the steer and provide the purchaser with meat.
For this reason, if the steer dies before being ritually slaughtered, the purchaser suffers the loss.

קניין הלכות מכירה פרק ט
Kinyan Mechirah Chapter 9