Gemara

(Rav Kahana): One can stipulate not to inherit an inheritance that is only mid'Rabanan (for Rava's reason);

(Rava): If an enactment was made to benefit a person, in a case like this, he can say 'I do not want the enactment.'

Rava refers to cases like Rav Huna's law;

(Rav Huna): A woman can tell her husband 'do not feed me, and I will not work for you' (because it was enacted for her benefit that he feed her and she will work for him).

Bava Kama 7b (R. Acha bar Yakov): A loan should be paid from Beinonis (at the current price). If the creditor requests a larger quantity of Ziburis, the borrower can stipulate that he receive it at the higher price of Nisan.

8a (Beraisa): If a debtor sold (Idis, Beinonis and Ziburis) to Levi, Levi is in place of the seller. (Victims, creditors and ex-wives collect from what they are entitled to, i.e. Idis, Beinonis and Ziburis, respectively.)

This is when he bought the Idis last.

Question: If so, everyone should collect from Idis!

Answer: Levi says 'it is is an enactment to help the buyer that we do not collect from Meshubadim (land that the debtor sold) when there are Bnei Chorin (land that he still owns). I do not want the enactment!'

Kesuvos 109b (Mishnah - Admon): If Levi went overseas and returned, and the path to his field was lost, he gets a short path to his field;

Chachamim say, he must pay whatever his neighbor charges for a path, or else he must fly (i.e. he may not walk) to his field.

(Rava): The case is, Levi was surrounded by four neighbors, and one person bought all their land. Admon says, in any case the path to his field is with the buyer! Chachamim say, the buyer says 'accept my terms. If you protest, I will return the deeds to the original owners (undo the sales), and you will be unable to force them to give you a path.'

Rishonim

The Rif and Rosh (Kesuvos 41a) bring the Gemara in Bava Basra

Rif (Bava Kama 3a): We conclude that it is is an enactment to help the buyer that creditors collect from the last land sold. The buyer can say 'I do not want the enactment!'

Rosh (Bava Kama 1:3): The buyer can say 'I do not want the enactment!' In such a case, we claim this on his behalf.

Rambam (Hilchos Malveh 19:6): If a buyer bought the Idis last, everyone collects from what they normally get. It is is an enactment to help the buyer that we collect from the last land sold. (In this case) the buyer does not want this enactment!'

Mishneh l'Melech (Hilchos Gerushin 9:27 b'Sof): Chachamim decreed not to allow the scribe and one other witness to sign a Get. However, if Igun will result (there is no other way to permit her to remarry, e.g. two heard a man in a pit authorize them to write a Get), we permit it with Edei Mesirah (witnesses who see the Get given). This is because Edei Mesirah are primary, and Edei Chasimah (the witnesses who sign it) are a mere enactment for her benefit (in case the Edei Mesirah go away). Here, the enactment harms her, so she can say that she does not want it.

Hagahos Ashri (Kesuvos 13:13): A case occurred in which Reuven and Shimon were partners in a house, and each owned Kedei Chalukah (a proper amount, so that he cannot refuse to divide). Shimon bought Levi's share in the house, which was less than Kedei Chalukah. Shimon said 'I do not want the enactment that allows me to demand a share in one place. Rather, I want in two places. If so, one will be less than Kedei Chalukah, so Reuven cannot force me to divide. I come with Levi's rights. Just like he could have vetoed the division, I may. If not, I will return my document of purchase to him!' My Rebbi said that his threat is empty, until he returns the document. We honor such a claim (I will return the document) only to keep what one has and annul the other's claim, like in Bava Kama 8a. Here, Reuven can say 'if you sell to Levi, I have first rights to buy, for I am the neighbor.' I disagree, for a neighbor has no more right to buy than the original owner. Also, a buyer gets all rights that the seller had. It is not a privilege to have less than Kedei Chalukah! Even though Shimon wants it, his opinion in Batel.

Hagahos Maimoniyos (Sefer Kinyan, Teshuvah 14): In Bava Kama, R. Acha taught that if a creditor requests a larger quantity of Ziburis, the borrower can stipulate that he receive it at the higher price of Nisan. Why can't he decline the enactment to collect Beinonis? The Riva answered that one can decline an enactment only when there is no loss to others, and he is Muchzak. A woman can tell her husband 'do not feed me, and I will not work for you' because she is Muchzak in her earnings. I would answer that the enactment of Beinonis for lenders was for the borrower's benefit, lest people be reluctant to lend. We honor a claim 'if not, I will return the document' only to let someone keep what he has.

Poskim

Shulchan Aruch (CM 119:5): If a buyer bought the Idis last (Rema - or at the same time), he is in place of the seller (everyone collects from what they normally get).

Beis Yosef (DH v'Chosav Od): Even if he does not say 'the enactment was to help me. I do not want the enactment!', the Nimukei Yosef (Bava Kama 3a DH b'Nezikim) says that anything one can do, we consider it as if he did it, even though it seems that Rashi (Kesuvos 109b DH Iy) disagrees (regarding one who lost the path to his field).

Beis Yosef (171:18 DH Kosav ha'Mordechai): The Mordechai (Bava Basra 569) says that if Reuven's share in a Chatzer or house is less than Kedei Chalukah, but his partner's share is Kedei Chalukah, he can force him to divide. If one says 'I do not want the enactment that Chachamim made for me (that I can refuse to divide)', we heed him.

Shulchan Aruch (CM 138:1): If two were holding one Kli, and each says 'it is mine', each swears (while holding a Chefetz, i.e. a Kodesh item such as a Sefer Torah or Tefilin) that he owns (at least) part, and he does not own less than half, and they divide it. If one said to the other 'swear and take it all', we heed him. If the other did not want to swear, they divide it without an oath.

Gra (6): One can decline an enactment for his benefit. Instead of swearing to receive, he allows the other to swear Heses. In this case, since the other must swear while holding a Chefetz for half, through Gilgul (when one must swear, his opponent can force him to include in the oath also other matters) he must swear about all of it while holding a Chefetz.

SMA (87:30): One can decline an enactment for his benefit, for we are not Mezakeh (acquire) for someone against his will.

Shulchan Aruch (EH 112:12): If a man died and left land and Metaltelim, and the land is too little (to feed the girls until they mature) and with the Metaltelim it is big (enough), the boys inherit.

Chelkas Mechokek (25): Why can't the girls say 'we decline the Ge'onim's enactment that we are fed from Metaltelim', so the estate will be small and the boys will not inherit? Perhaps there is no reason to judge from the girls' perspective. From the boys' perspective, the estate is big!

Beis Shmuel (25): The Shulchan Aruch holds that the enactment was for the girls, and all the more so for the boys, therefore, the girls cannot decline it.

Hafla'ah (Kuntres Acharon on Kesuvos 100:2): If a man sold land and left Metaltelim, I am unsure whether or not his wife can say 'I do not want the enactment that allows collecting a Kesuvah from Metaltelim. (I want to take the land.)' (I do not ask according to the Rosh, who says that nowadays she collects without the enactment, for people rely on Metaltelim like land). It is not clear whether this enactment was also to benefit the buyer, like the Beis Shmuel's answer. I answer that if one declines an enactment, he must accept the Torah law. He cannot partially decline. The opinion that Kesuvah is mid'Oraisa agrees that Chachamim enacted to collect only from land. Since the Torah allows collecting from Metaltelim, she may not take Meshubadim if there is Bnei Chorin. (This was surely to benefit buyers!)