In regard to oil that remains in the bottle, the Biur Halachah (677:4, d.h. hatzorich) writes, that even if one brought the bottle of oil specifically to be used for Chanukah, the oil that remains doesn’t have the same din as oil that remains in the menorah.
The question is, why we don’t we say הזמנה מילתא היא, setting something aside is significant, and say that since the bottle of oil was purchased to be used for Chanukah, it has been set aside for the mitzvah and one is forbidden to benefit from it?
The Taz (665:1) writes in regard to an esrog, that on the first day of Succos even before one uses it, it isהוקצה למצוותו, set aside for a mitzvah. The Beis Yosef citing the Hagaos Ashri writes, that if one sets aside aravos to be used for hoshanos and he hasn’t yet bound them, one may use them for alternative uses as,הזמנה לאו מילתא היא, setting something aside is not significant. The Taz explains, that the Beis Yosef is not a contradiction. By hoshanos there is an act of binding to do, therefore, as long as one hasn’t bound them one hasn’t taken his mind off them (אקציה דעתיה), however, by the esrog there is nothing more to do, therefore, as soon as bein hashmoshas [twilight] arrives, they are set aside for the mitzvah, and one can no longer use it for something else. The question is, why don’t we say this by the leftover oil in the bottle?
The Pri Megodim (M.Z. s.k. 1) asks on the Taz from the Shulchan Aruch (653:2) which says that if one binds hoshanos together, however, he hasn’t yet picked them up to fulfill the mitzvah, they may be used for alternative purposes. Which seems to imply, even if one did an act, one can still change his mind and use the item for something else. The Pri Megodim ends off tzorich iyun, that the matter needs more clarification.
The Biur Halachah (665 d.h. sheharay huktzeh) cites the Taz and says that all the Achronim argue on the Taz as he is coming from an incorrect nussach [textual version] of the Hagaos Ashri. If in fact we don’t go like the Taz, then it’s understandable why the leftover oil in the bottle is not prohibited.
