Nedarim 12a - Tevel and Teruma

The Ran and Rosh argue why Teruma is considered a davar ha'asur and not a davar ha'nadur. The Ran says that it is because it is only assur to zarim but mutar to kohanim, so long as it is not assur to all it cannot be called a davar hanadur. The Rosh and Tosafos say that since it was previously assur as tevel, by making it Teruma he is creating heter rather than issur.
R' Yossi already pointed out on this blog in yevamos that the machlokes ran and rosh may very well be the machlokes rashi and tosfos - http://hearos.blogspot.com/2007/07/86.html

Regarding the opinion of the Rosh, Tosafos here, and Rashi in Yevamos - R' Moshe (dibros heara #67) asks the obvious question that the status of Teruma creates kedusha which is assur to zarim, whereas tevel doesn't have kedusha it is just an issur that is assur to everyone. R' Moshe explains based on the Rambam (ma'achalos asuros chapt. 10) that regarding the misah bidei shamayim of tevel, it is only an issur misah if it is tevel that has teruma mixed in it, but if it is tevel wheter the teruma has been removed and only has ma'aser sheini or ma'aser ani mixed in, there is no chiyuv misah. Based on this it seems that really the chumra of misah by tevel is because of the kedushas teruma that is in it, but since it is just mixed in, it is not as severe regarding other issues i.e. tumah. According to this approach the ba'alim by seperating Teruma are not being mechadesh the kedusha status of Teruma, rather the Teruma is already there and the ba'alim is simply identifying the area where the Teruma is thereby making the remaining parts mutar as chulin.

The opinion of the Ran is also difficult - obviously if one makes a neder on an object to be assur only to some people and not others, it is considered a davar hanadur. Why then is teruma considered davar ha'asur just because it is not assur to everyone? The ran earlier on the top of the page sheds some light on this. Anytime he makes a neder that explicitly excludes some people it still is a davar hanadur. But, if his language was all inclusive, but because a gezeiras hakasuv the result of calling it "teruma" would make it mutar to some and assur to others, it is a davar ha'asur since the Torah is deciding who should be assur and who should be mutar - v'dok.