Nedarim 13 - 14 - Matfis in a davar ha'nadur not a davar ha'asur

R' Moshe points out in one of his shiurim (Dibros siman 10 anaf 2) a very fundamental machlokes why a hatfasa is only binding in a davar ha'nadur and not a davar ha'asur. Although the gemara ultimately learns this out from pesukim on 14a, it is clear from the Rishonim that there is some element of sevara necessary to interpret the pesukim (otherwise we would have darshened that you can be matfis in a davar ha'asur from the word "isur" rather than using it for something else).

The Ran understands that hatfasa can only work to transfer or apply an issur that was implemented by a person, but cannot work to transfer an issur that was placed on an object by the Torah. The issur imposed by a person is inherently different than an issur imposed by the Torah, therefore when trying to place an issur on an object one can only be matfis in an issur that was originally created by a person, and not an issur that was originally created by the Torah.

The Rambam (Nedarim 1:8) writes that "one cannot turn something which is not basar chazir into basar chazir". R' Moshe explains that it would technically be possible to transfer even an issur Torah and place it on the object of the neder, because there isn't any fundamental difference between an issur imposed by a person and an issur imposed by the Torah. The only reason hatfasa doesn't work on isurei torah such as basar chazir, is that issurei torah are not separate entities from the metzius of the object. Meaning, it is not as if there is pig meat on which the Torah places a separate status of issur; rather the Torah identifies pig meat as being assur. It is the identification of the metzius of pig meat that is assur, and therefore is not transferable to another object that is not pigs meat. A neder on the other hand is a separate issur status that is placed on the object and is easily transferable to another object.

B'kitzur - According to the Rambam an issur is totally tied to the object itself and is therefore non-transferable. According to the Ran the issur is a separate entity that the Torah places on the object, but because it is not created by a person it is not transferable.

There is a discussion whether a neder can be made on something which is already assur min hatorah like pig meat. Although the gemara says that one can make a neder on a d'var mitzvah, the ran is shavuos says that one can make a neder that will result in violation of a bitul aseh (such as a neder on matzah), but a neder will not be binding on a lo ta'aseh (like on pig meat). R' Moshe suggests that this would be dependent on the machlokes Rambam and Ran. According to the Rambam that issur neder is a chiddush that one can place a new status on an object, is makes not difference whether the object is a metzius of heter or a metzius of issur, he can still place the neder on it. But according to the Ran that an issur torah is not fundamentally different than an issur neder (just that issur torah is not created by a person and therefore not transferable), it seems logical that once there is already a status of issur torah on an object one cannot place on top of it issur neder (since they are fundamentally the same it is like an issur on an issur or a neder on a neder, that is not chal).

k'darcho ba'kodesh - R' Moshe uses this as a foundation to explain many many things - too many for this blog!