1) OWNERSHIP OF AN OBJECT WHICH IS "ASUR B'HANA'AH"
OPINIONS: The RAN quotes the RASHBA who rules that when one who makes a Neder to prohibit an object of his own upon himself, anyone may come and take that object. The Rashba adds that the original owner may go to a Chacham to have his Neder repealed and thereby cause the object to revert back to his domain (if it has not yet been taken). However, as long as he has not repealed his Neder, anyone may take the object.
The Ran questions the assertion of the Rashba that the object reverts to the original owner's possession when the Neder is repealed. When one makes a Neder to prohibit an object upon himself, he is considered as though he makes the object Hefker. When he subsequently rescinds the Neder he is not empowered to rescind the implied Hefker, and therefore the object cannot revert to his ownership.
The Rashba obviously does not agree with the Ran's reasoning that the implied Hefker is independent of the Neder, and thus even when the Neder is rescinded the Hefker remains. Rather, the Rashba apparently maintains that that any object which is Asur b'Hana'ah automatically leaves the possession of its owner. This opinion is not consistent with the Ran's opinion (42b and 47a) that a son inherits his father's property even when the father had previously prohibited his possessions to his son. According to the Ran's opinion there, it is evident that he maintains that not only does one not surrender ownership of an object which is Asur b'Hana'ah, but one can even assume possession of such an object.
The question of ownership of an object which is Asur b'Hana'ah arises numerous times throughout the Gemara and the Rishonim, and there are many differences of opinion in the matter. Below is a summary of the basic opinions and some of the primary proofs for those opinions.
(a) TOSFOS in Kidushin (56b) quotes the Mishnah that states that when one is Mekadesh a woman with a fruit of Orlah (which is Asur b'Hana'ah) the Kidushin is invalid. Tosfos assumes that the reason why the Kidushin is invalid is that the Orlah is worthless because it is Asur b'Hana'ah, and thus the man gave nothing of value to the woman. Tosfos asks that the Kidushin should be valid, because although one may not benefit from Orlah in an ordinary manner, one may benefit from Orlah in an unconventional manner ("she'Lo k'Derech Hana'aso"), and thus the Orlah is not worthless and the Kidushin should be valid.
Tosfos clearly implies that there does exist a quality of ownership of an object which is Asur b'Hana'ah. If the man does not own the fruit of Orlah, he certainly could not use it for Kidushin (because even if it has value, it is not his). It must be that he has some quality of ownership of the fruit of Orlah.
Some Rishonim attempt to limit the opinion of Tosfos to only Isurei Hana'ah which have a potential value, while those which are absolutely worthless cannot be said to be owned at all. This limitation seems to be supported by the RAN in Avodah Zarah (41a) who writes that although an object of Avodah Zarah is Asur b'Hana'ah and cannot be owned, one may assume ownership of the Avodah Zarah of a Nochri because it has potential value -- a Nochri could be Mevatel the Avodah Zarah and render it permissible. The Ran's words there support the opinion that potential value suffices to allow ownership.
(b) The RITVA in Sukah (35a) quotes an opinion that an Esrog of Orlah may not be used for the Mitzvah since it is not considered to belong to the bearer, and the Mitzvah of Esrog requires that it belong to him ("Lachem"). The Ritva disagrees with this opinion and states that since others cannot claim ownership of it, it is considered his. (The Ritva offers a different reason for its disqualification.) The Ritva clearly is of the opinion that Isurei Hana'ah may be owned.
(c) However, the prevalent opinion is that Isurei Hana'ah defy ownership. When RASHI in Pesachim (6a) explains the statement of the Gemara that Chametz on Pesach is not in the domain of its owner, he writes that "it does not belong to him." RASHI in Eruvin (30a) writes that a bread of Isurei Hana'ah may not be used to make an Eruv, because, according to most commentaries, such bread cannot be considered "owned" by him since it is Asur b'Hana'ah. (For a further discussion of this subject, see BEIS HA'LEVI 1:48 and MARCHESHES 1.)