Gemara

Question (Rav Chiya bar Avin): If Reuven said to Shimon 'my loaf is forbidden to you', and then he gave it to him, what is the law?

Does 'my loaf' mean 'as long as it is in my possession'?

Or, since he said 'to you', it is like Hekdesh to Levi, and it is always forbidden!

Answer (Rava): Clearly, it is forbidden to Shimon. If it were permitted, why did he say 'my loaf'? Will you say that he forbade it only if Shimon will steal it?!

Rejection (R. Chiya bar Avin): Really, it is permitted. 'My loaf' forbids it if Reuven will invite Shimon to eat from it.

Question (against Rava - Beraisa): If Levi asked David 'lend to me your cow' and David responded 'my cow is forbidden to you if I own another one':

If it was found that David owned another; his cow is forbidden to Levi in David's lifetime. If David died or it was given to Levi for a gift, it is permitted.

Answer (Rav Acha brei d'Rav Ika): David gave it to someone else, and that person gave it to Levi.

Support (Rav Ashi): The Beraisa says 'it was given to him', not 'he gave it.'

42a: Suggestion #1: Rav (and Shmuel) hold that one can forbid his property even for after it leaves his possession. R. Yochanan (and Reish Lakish) disagree.

Objection #1: Surely, all agree that one can forbid! If not, they would have argued in a case when he said 'this property', and all the more so if he said 'my property'!

Objection #2 (Mishnah): If Reuven told his son 'you may not benefit from me', his son inherits him. If he said '...in my life nor after my death', his son does not inherit him. (I.e. he may not benefit from the property, even after it left Reuven!)

We can answer Objection #2 by saying that there is different, for he said 'in my life and after my death.' However, Objection #1 still stands.

Suggestion #2: All agree that 'this property' forbids it even after it leaves him. They argue about when he says 'my property.'

Objection: Surely, all agree that if he said 'my property', it remains forbidden!

(Mishnah (46a)): If Levi told David 'I will not enter your house or buy your field', and David died or sold them, they are permitted. If he forbade 'this house' or 'this field' and David died or sold them, they are forbidden.

Conclusion: Rather, R. Yochanan discusses one who says 'my property.' Rav discusses one who says 'this property'. They do not argue.

47a (Avimi) Question: If Reuven said to Shimon 'you may not enter this house', and Reuven died or sold it, is Shimon permitted? I.e., can one forbid property for even after it does not belong to him?

Answer (Rava - Mishnah): If Reuven told his son 'you may not benefit from me', when Reuven dies, his son inherits him. If he said '...during my life and after my death', his son does not inherit him.

This shows that one can forbid something for even after it is not his.

Rishonim

The Rif and Rosh (4:4 and 5:2) bring the Gemara verbatim.

Rosh (4:4): The Halachah follows Rava against R. Chiya bar Avin, because Rav Ashi said that the Beraisa supports Rava.

Rosh (5:2): Abaye (our texts - Avimi) asked about when Reuven said 'you may not enter this house during my life and after my death.' Therefore, he could not dispel Rava's answer (like above, 42a) by saying that there is different.

Ran (47a DH Bo'i): Avimi did not learn from the Mishnah (46a), for there he forbids to himself. Perhaps one cannot forbid to another property after it is not his, just like he cannot forbid another's property to another! Above, we said that forbidding 'this' does not forbid to another permanently, unless he said 'even after death.' Avimi rejects this. If one can forbid, surely 'this' suffices!

Beis Yosef (YD 216 DH veha'Rosh): The Rosh holds that the Gemara proved that Rav and R. Yochanan surely agree that 'this' forbids permanently, but perhaps this is not the Halachah.

Rambam (Hilchos Nedarim 5:4): If Shimon said to Levi 'my loaf is forbidden to you', even if he then gave it to him, it is forbidden to him. If Shimon died and Levi inherited it, or Shimon gave it for a gift to Ploni, it is permitted to Levi. Shimon forbade only 'my loaf', and now it is not his.

Rambam (5): If he said 'these Peros are forbidden to you', and did not say 'my Peros', even if he sold them or died and Ploni inherited them, they are forbidden to Levi. If Shimon forbids his property on another, it is forbidden even after it leaves Shimon's Reshus, unless he said 'my property or Peros or house', which connotes that it is forbidden only as long as it is in my Reshus.

Rambam (6): If he told his son 'you may not benefit from me', or swore that his son may not benefit from him, his son inherits him. It is as if he said 'my property is forbidden to you.' If he specified 'in my life and after my death', his son does not inherit him. It is as if specified 'this property is forbidden to you.'

Rambam (Hilchos Me'ilah 4:10): If Reuven told Shimon 'my loaf is Hekdesh to you' and later gave it to him, Shimon transgresses Me'ilah when he gives it away, for it was not forbidden to Reuven.

Poskim

Shulchan Aruch (YD 216:7): If Reuven told Shimon 'this house is Konam; you may not enter it' and Reuven died or sold it to Ploni, it is forbidden to Shimon.

Gra (16): This is unlike the Rosh, who says that the question is only when he says 'during my life and after my death.'

Shach (18): It seems that if the house fell and was rebuilt it is permitted, like in Sa'if 5 (which says so about one who forbade himself to 'this house').

Shulchan Aruch (ibid.): If Reuven forbade his property to Shimon, it is forbidden even after it leaves Reuven's Reshus. However, if he said 'my house is Konam; you may not enter it' and he died or sold or gave it to Ploni, Shimon is permitted. If he told his son 'you may not benefit from me', or swore that his son may not benefit from him, his son inherits him. It is as if he said 'my property is forbidden to you.' If he specified 'in my life and after my death', his son does not inherit him. It is as if specified 'this property is forbidden to you.'

Bach (DH v'Rabbeinu): Surely, one can forbid his property even after it leaves him only if he began the Isur when he owned it.