Gemara

36a: A case occurred in which Ploni was obligated to pay Shimon a half-Dinar for damage. Shimon said 'give it to the poor'. He later retracted and asked to get it.

Rav Yosef: You cannot retract. The poor already acquired it! Even though there are no poor people here, I acquired on behalf of poor people elsewhere.

68b (Mishnah): If one stole an animal and made it Hekdesh and then slaughtered it, he pays Kefel, but not four and five.

(Reish Lakish): The case is, the owner made it Hekdesh in the thief's house.

Question: R. Yochanan taught that if something was stolen, and the owner did not despair, the thief cannot make it Hekdesh, for it is not his. Also the owner cannot be Makdish it, for it is not in his Reshus (under his control)!

Answer: Reish Lakish holds like Tzenu'im (pious people, who were concerned lest people taking Shemitah produce mistakenly take Revai fruits of a tree in its fourth year, which must be eaten in Yerushalayim or redeemed).

(Mishnah #1): Tzenu'im would say 'whatever (Revai) was gathered from my field is redeemed on these coins.' (Just like one can redeem something not in his Reshus, he can be Makdish something not in his Reshus!)

R. Yochanan holds like a Stam Mishnah that argues with Mishnah #1.

(Mishnah #2): One who steals from a thief does not pay Kefel. If he then slaughters or sells it, he does not pay four and five.

He doesn't pay the thief, for it is not the thief's. He doesn't pay the owner, for it was not in his Reshus. (Similarly, neither can make it Hekdesh.)

R. Yochanan rules like Mishnah #2 against Mishnah #1 because a verse supports it - "if a man will make his house Hekdesh." One can be Makdish only things like his house, which is in his Reshus.

Bava Metzia 6b: A case occurred in which Reuven and Shimon were fighting over a bathhouse. Each said 'it is mine.' Reuven was Makdish it. R. Chananya, R. Oshaya and all the Chachamim stopped using it.

7a (R. Chiya bar Avin): If one was Makdish property that he cannot obtain through Beis Din, it is not Hekdesh.

Question: This implies that if he can obtain it through Beis Din, he can be Makdish it, even before receiving it. This is unlike R. Yochanan!

Answer: The bathhouse is not movable. Rather, it is land. If one can obtain land through Beis Din, it is as if it is in his Reshus.

Bava Basra 88a: A case occurred in which Reuven brought gourds to Fum Nahara to sell; everyone took one each (intending to buy it; he thought that they were stealing). He said 'they are Hekdesh'!

Rav Kahana: One cannot make Hekdesh something that is not his.

This is only if the price is known. (The buyers acquired them; they merely owed money.) . If not, they still belonged to Reuven; they are now Hekdesh.

Rishonim

The Rif (Bava Kama 27a) brings R. Yochanan's law.

Rif and Rosh (Bava Metzia 3b and 1:12): If Ploni was Makdish land that he can obtain through Beis Din, it is Hekdesh. If he was Makdish Metaltelim that he can obtain through Beis Din, it is not Hekdesh. This is when the one holding it denies that it is Ploni's, and Ploni must force him through Beis Din. If a Shomer is guarding a deposit and will return it when asked, this is like land; it is in the owner's Reshus. Also a Gaon said so.

Rosh: Also Bava Metzia 88a teaches this. People took gourds from a seller, and the seller was Makdish them. They became Hekdesh (if the price was not known, for the buyers did not acquire, and they did not intend to steal).

Rambam (Hilchos Erchin 6:22): One cannot be Makdish something not in his Reshus. If Ploni had a deposit with Shimon who denied having it, Ploni cannot be Makdish it. If Shimon did not deny it, it is in Ploni Reshus whenever it is.

Rambam (23): This refers to Metaltelim. If Shimon stole Ploni's land and denies it, if Ploni can force him to return it through Beis Din, Ploni can be Makdish it even though he did not get it back yet. The land itself is in his Reshus.

Rambam (24): If Shimon stole from Levi, and Levi did not despair, Shimon cannot make it Hekdesh, for it is not his. Levi cannot be Makdish it, for it is not in his Reshus. The same applies to all similar cases.

Poskim

Shulchan Aruch (YD 258:7): One cannot be Makdish something not in his Reshus. If Ploni had a deposit with Shimon who denied having it, Ploni cannot be Makdish it. If Shimon did not deny it, it is in Ploni's Reshus whenever it is. This refers to Metaltelim. If Shimon stole Ploni's land and denies it, if Ploni can force him to return it through Beis Din, Ploni can be Makdish it even before getting it back. The land itself is in his Reshus. If Shimon stole from Levi, and Levi did not despair, Shimon cannot make it Hekdesh, for it is not his. Levi cannot be Makdish it, for it is not in his Reshus. The same applies to all similar cases.

Beis Yosef (DH u'Mah she'Chosav Rabeinu): Chidushei ha'Ran (Bava Metzia 7a DH Ha) derives that one cannot be Makdish a loan that others owe him, for it is not in his Reshus.

Rema: If David has a security from Moshe for a loan, Moshe cannot be Makdish the amount corresponding to the loan. He can be Makdish its value above the loan. Some say that he cannot be Makdish even the excess.

Darchei Moshe (4): The Mordechai (Bava Basra 565) rules that one can be Makdish a loan in another's hand, and all the more so a deposit, but not what was stolen or given for a deposit. However, one can be Makdish the value of the security above the loan. R. Yerucham (Adam v'Chavah 167a) says that he cannot be Makdish even the excess. The Mordechai (Bava Metzia 419,420) says that whenever the law is 'whoever overpowers keeps it', if one seized it, he can be Makdish it while he has it, even though the other may seize it back. Hekdesh before it came to his hand is invalid.

Shulchan Aruch (8): If one was owed a debt and said 'it will be Hekdesh or Tzedakah', his words do not take effect. ): If he said 'what Ploni owes me, I will be Makdish it or give it to Tzedakah wen I get it', he must fulfill his words when he gets it. Even when he said 'it will be Tzedakah', if he said so in front of the borrower and the Gabai (overseer of Tzedakah) or the seven leaders of the city (Rema - or an important person in the city), the Gabai acquired it through the law of Ma'amad Sheloshtam. It is Tzedakah, and he may not change it.

Shach (18): We are the Yad of Aniyim; it is as if the money came to them. He cannot even permit his vow (Rashba 656).