Halacha

הלכה א
הַמּוֹכֵר שָׂדֶה לַחֲבֵרוֹ בְּאֶלֶף זוּז וְנָתַן לוֹ מִקְצָת הַדָּמִים וְהָיָה יוֹצֵא וְנִכְנָס וְתוֹבֵעַ שְׁאָר הַדָּמִים אֲפִלּוּ לֹא נִשְׁאַר לוֹ אֶלָּא זוּז אֶחָד לֹא קָנָה הַלּוֹקֵחַ אֶת כֻּלָּהּ אַף עַל פִּי שֶׁכָּתַב הַשְּׁטָר אוֹ הֶחֱזִיק:
כסף משנה
1.
The following rules apply when a person sold his field to a colleague for 1000 zuz, the purchaser paid a portion of the funds, and the seller was repeatedly demanding payment of the remainder. Even if there was only one zuz remaining unpaid, the purchaser does not acquire the entire field. This applies even if the seller had a deed of sale composed or the purchaser manifested ownership.

הלכה ב
חָזַר בּוֹ הַלּוֹקֵחַ. יַד הַמּוֹכֵר עַל הָעֶלְיוֹנָה. רָצָה אוֹמֵר לוֹ הֵילָךְ מְעוֹתֶיךָ אוֹ קְנֵה מִן הַקַּרְקַע כְּנֶגֶד הַמָּעוֹת שֶׁנָּתַתָּ לִי וְנוֹתֵן לוֹ מִן הַזִּבּוּרִית שֶׁבָּהּ. וְאִם חָזַר הַמּוֹכֵר יַד הַלּוֹקֵחַ עַל הָעֶלְיוֹנָה רָצָה אוֹמֵר לוֹ תֵּן לִי מְעוֹתַי אוֹ תֵּן לִי קַרְקַע כְּנֶגֶד מְעוֹתַי וְנוֹטֵל מִן הַיָּפֶה שֶׁבָּהּ. וְאִם לֹא הָיָה יוֹצֵא וְנִכְנָס וְתוֹבֵעַ קָנָה הַלּוֹקֵחַ אֶת כֻּלָּהּ וְאֵין אֶחָד מֵהֶן יָכוֹל לַחְזֹר בּוֹ. וּשְׁאָר הַדָּמִים עָלָיו כִּשְׁאָר הַחוֹבוֹת:
כסף משנה
2.
In the above situation, if the purchaser retracts, the seller is given the upper hand. If he desires, he can tell the purchaser: "Here is your money," or he can tell him: "Acquire a portion of the land equivalent to the money that you paid me." In this instance, he gives the purchaser the land of least value.
If the seller retracts, the purchaser is given the upper hand. If he desires, he can tell the seller: "Give me my money," or he can tell him: "Give me a portion of the land equivalent to the money that I paid." In this instance, he takes the choicest portion of the land.
If the seller was not repeatedly demanding payment of the remainder, the purchaser acquires the entire property. Neither can retract. The remainder of the money unpaid at the time of sale is considered as any other debt.

הלכה ג
מָכַר שָׂדֵהוּ מִפְּנֵי רָעָתָהּ. אַף עַל פִּי שֶׁהוּא יוֹצֵא וְנִכְנָס וְתוֹבֵעַ שְׁאָר הַדָּמִים קָנָה הַכּל. וְאֵין הַלּוֹקֵחַ יָכוֹל לַחְזֹר בּוֹ. שֶׁזֶּה שֶׁתּוֹבֵעַ וְרוֹדֵף לֹא מִפְּנֵי שֶׁעֲדַיִן לֹא גָּמַר וְהִקְנָה. אֶלָּא כְּדֵי שֶׁלֹּא יַחְזֹר בּוֹ הַלּוֹקֵחַ:
כסף משנה
3.
If a person sold his field because it was of inferior quality, even when the seller repeatedly demands payment of the remainder of the money, the purchaser acquires the entire property, and he is not entitled to retract. For the reason the seller is pursuing the purchaser and demanding payment is not that he has not agreed to transfer ownership, but to prevent the purchaser from retracting.

הלכה ד
וְכֵן הַדִּין בְּמוֹכֵר מִטַּלְטְלִין אַף עַל פִּי שֶׁמָּשַׁךְ הַלּוֹקֵחַ הַפֵּרוֹת וְהוֹצִיאָן לִרְשׁוּתוֹ וְהַמּוֹכֵר נִכְנָס וְיוֹצֵא עַל שְׁאָר הַדָּמִים לֹא קָנָה. וְיַד הַחוֹזֵר בּוֹ עַל הַתַּחְתּוֹנָה כְּמוֹ שֶׁבֵּאַרְנוּ אֶלָּא אִם כֵּן מָכַר מִפְּנֵי רָעַת מִמְכָּרוֹ וַהֲרֵי זֶה קָנָה הַכּל:
כסף משנה
4.
The same law applies with regard to the sale of movable property. Although the purchaser draws produce after him and brings it into his domain, if the seller repeatedly demands payment of the remainder of the money, the purchaser does not acquire the goods. The person who retracts has the lower hand, as has been explained above, unless the seller sold the goods because of their inferior quality. In such an instance, the purchaser acquires all the goods.

הלכה ה
לָקַח שְׁוֵה מֵאָה בְּמָאתַיִם וְהַמּוֹכֵר יוֹצֵא וְנִכְנָס לִתְבֹּעַ שְׁאָר הַדָּמִים. הֲרֵי זֶה סָפֵק אִם הוּא כְּמוֹכֵר שָׂדֵהוּ מִפְּנֵי רָעָתָהּ וְאֵינוֹ תּוֹבֵעַ אֶלָּא מִפְּנֵי שֶׁמָּכַר בְּיוֹתֵר. אוֹ אֵינוֹ כְּמוֹכֵר שָׂדֵהוּ מִפְּנֵי רָעָתָהּ וְזֶה שֶׁתּוֹבֵעַ מִפְּנֵי שֶׁעֲדַיִן לֹא גָּמַר לְהַקְנוֹתוֹ עַד שֶׁיִּקַּח כָּל הַדָּמִים. לְפִיכָךְ הָרוֹצֶה לַחְזֹר מִשְּׁנֵיהֶם אֵינוֹ יָכוֹל לַחְזֹר. וְאִם תָּפַס הַמּוֹכֵר מִמִּקָּח שֶׁמָּכַר כְּנֶגֶד הַמָּעוֹת שֶׁנִּשְׁאֲרוּ לוֹ אֵין מוֹצִיאִין מִיָּדוֹ:
כסף משנה
5.
When a person buys a field worth 100 zuz for 200, and the seller repeatedly demands payment of the remainder of the money, the status of the transaction remains an unresolved question: Is the person considered to be one who sold his field because of its inferior quality, and he is demanding payment only because he sold it for more than its worth? Or he is not considered to be one who sold his field because of its inferior quality, and he is demanding payment because he did not decide to complete its sale until he received all the money?
Because the matter is unresolved, neither party is allowed to retract. If the seller seizes possession of a portion of the property that was sold equivalent to the worth of the money left unpaid, it should not be expropriated from his possession.

הלכה ו
הַקּוֹנֶה דָּבָר מֵחֲבֵרוֹ וְנָתַן לוֹ הַדָּמִים וְטָעָה בְּמִנְיַן הַמָּעוֹת. וּלְאַחַר זְמַן תְּבָעוֹ הַמּוֹכֵר וְאָמַר לוֹ מֵאָה שֶׁנָּתַתָּ לִי אֵינָם אֶלָּא תִּשְׁעִים. נִקְנָה הַמִּקָּח וּמַחֲזִיר לוֹ הָעֲשָׂרָה אֲפִלּוּ אַחַר כַּמָּה שָׁנִים. בֵּין בְּקַרְקַע בֵּין בְּמִטַּלְטְלִין:
כסף משנה
6.
When a person acquires something from a colleague and pays him money, but errs in the amount of money he gave him the transaction is valid. If afterwards the seller lodges a claim against the purchaser, saying: "You were supposed to pay me 100 zuz, and you paid me only ninety," the transaction is still viable. The purchaser must, however, pay the additional ten zuz.
The above applies even though several years have passed since the transaction and applies with regard to the sale of both landed property and movable property.

הלכה ז
הָאוֹמֵר לַחֲבֵרוֹ כְּשֶׁאֶמְכֹּר שָׂדֶה זוֹ הֲרֵי הִיא מְכוּרָה לְךָ מֵעַכְשָׁו בְּמֵאָה זוּז וְקָנָה מִיָּדוֹ עַל כָּךְ וּלְאַחַר זְמַן מְכָרָהּ לְאַחֵר בְּמֵאָה. קָנָה הָרִאשׁוֹן. מְכָרָהּ בְּיוֹתֵר עַל מָנֶה קָנָה אַחֲרוֹן. שֶׁלֹּא אָמַר לוֹ אֶלָּא כְּשֶׁאֶמְכֹּר שֶׁיִּהְיֶה מוֹכֵר מִדַּעְתּוֹ תְּחִלָּה וְזֶה לֹא הָיָה רוֹצֶה לִמְכֹּר וְלֹא מָכַר אֶלָּא מִפְּנֵי הַתּוֹסֶפֶת שֶׁהוֹסִיף זֶה עַל שָׁוְיוֹ. וְנִמְצָא כְּמִי שֶׁנֶּאֱנַס וּמָכַר:
כסף משנה
7.
The following rules apply when a person tells a colleague: "If I sell this field, it will be sold to you retroactively from the present time for 100 zuz" and confirms this with a kinyan. If the seller later sells it to another person for 100 zuz, it is acquired by the first person.
If he sells it for more than that amount, the second person acquires it. The rationale is that by saying: "If I sell," he meant "If I sell it with the same approach that I have now." And this person did not want to sell it; he sold it only because of the additional amount that the other person added on. It is as if he were forced to sell.

הלכה ח
אָמַר לוֹ כְּשֶׁאֶמְכְּרֶנָּה הֲרֵי הִיא קְנוּיָה לְךָ מֵעַכְשָׁו כְּמָה שֶׁיָּשׁוּמוּ אוֹתָהּ בֵּית דִּין שֶׁל שְׁלֹשָׁה. אֲפִלּוּ עַל פִּי שְׁנַיִם מִן הַשְּׁלֹשָׁה. אָמַר לוֹ כְּמוֹ שֶׁיֹּאמְרוּ שְׁלֹשָׁה עַד שֶׁיֹּאמְרוּ הַשְּׁלֹשָׁה. וְכֵן אִם אָמַר לוֹ כְּמוֹ שֶׁיָּשׁוּמוּ אוֹתָהּ בֵּית דִּין שֶׁל אַרְבָּעָה עַד שֶׁיָּשׁוּמוּ הָאַרְבָּעָה כֻּלָּן וְיַסְכִּימוּ וְיִמְכֹּר לְאַחֵר כְּמוֹ שֶׁיַּסְכִּימוּ. וְאַחַר כָּךְ יִקְנֶה הָרִאשׁוֹן. שָׁמוּ אוֹתָהּ שְׁלֹשָׁה אוֹ אַרְבָּעָה וְאָמַר הַמּוֹכֵר עַד שֶׁיָּבוֹאוּ שְׁלֹשָׁה אֲחֵרִים אוֹ אַרְבָּעָה וְיָשׁוּמוּ אֵין שׁוֹמְעִין לוֹ שֶׁהֲרֵי קָנוּ מִיָּדוֹ תְּחִלָּה שֶׁמָּכַר מֵעַכְשָׁו:
כסף משנה
8.
If the seller told the first purchaser: "If I sell this field, it will be sold to you retroactively from the present time for the price to be evaluated by a court of three," the price need be agreed on by only two of the three.
If he said: "As dictated by three," all three must agree. If he said: "As evaluated by a court of four," all four must agree. For the sale to the first purchaser to take effect, the judges must agree on the price, and he must sell it to another person as they agreed. Afterwards, the first purchaser acquires the property.
If three or four people evaluate it and the seller says: "I will not sell it until three or four others evaluate it," we do not heed him. For he confirmed his commitment to sell retroactively with a kinyan.

קניין הלכות מכירה פרק ח
Kinyan Mechirah Chapter 8