The machlokes (see yesterday’s post) between Rashi and Tosfos whether hefker simply means others have the right to take an object or whether it is an actual hakna’ah out of the owner’s reshus helps explain another machlokes Rashi and Tosfos which we discussed Pesach time. Rashi (Pesachim 4) writes that bitul chameitz is a fulfillment of the mitzvah of tashbisu. Tosfos, however, disagrees and says that bitul is simply a way of declaring chameitz to be hefker, eliminating the need to dispose of it and fulfill tashbisu.
The Ketzos explains that Rashi could not possibly explain bitul using the model of hefker. According to Rashi, hefker is simply a pledge which grants others the right to take an object. Even if one is mafkir chameitz, as long as no one has yet taken it, it is still considered in the possession of its original owner who would therefore be in violation of bal year’eh. Only because bitul is considered an act of destroying chameitz does is have any effect.
A similar machlokes with respect to chameitz that may hinge on this issue is whether a shliach can be appointed to do bitul. The Ran writes that shlichus does not work because one cannot appoint a shliach to be mafkir one’s property. The Beis Yosef disagrees and distinguishes between bitul chameitz and other forms of hefker. If hefker is a form of neder (coming back to the Rambam’s formulation), then just as one cannot appoint a shliach to take a neder on one’s behalf, one cannot appoint a shliach to be mafkir on one’s behalf. But if bitul is not a form of hefker (or if it is a different type of hefker that functions more like a real kinyan), then there is more of a reason to allow shlichus in that context.