Kinyan Kesef shel Achsenai

There is an interesting halacha by the mitzva of Ner Chanuka called achsanai. Simply put this means that if someone is a guest at someone else's house they are able to be yotzei their chiyuv hadlaka by giving the ba'al habayis a perutah. (mishtateif b'perutah). Leaving aside the issue of how this fits into the chakirah of whether Ner Chanukah is a chovas hagavrah or chovas habayis, there is a more fundemental question that needs to be answered. What is the mechanism that Chazal were trying to accomplish thru the takkanah of mishtateif b'perutah? The pashtus is that the perutah serves as a kinyan kesef, meaning that by giving over a perutah you buy a chelek of the oil. The question on this is that we pasken that even though min hatorah money creates a kinyan on mitaltilin, m'derabanan it doesn't work. Therefore, how could you buy the oil with a perutah if m'derabanan the kinyan is not a kinyan?

I heard this question from HaRav Shmuel Kaplan shlita. . Rav Kaplan said one could argue that by mitzvos, money is koneh. However, the problem with saying this is that the G"RA doesn't pasken like that. (It is a machlokes haposkim and the GRA holds that except for 4 cases listed in Eiruvin money isn't koneh by mitzvos. Rav Kaplan gave a different hesber to explain why by Ner Chanukah, the perutah would create a kinyan. He said that what would happen if you buy a candle from someone and the seller puts it on the table and lights it for you. You then proceed to use the candle without ever touching it. Would you have a right to say that since you never picked up the candle you never made a kinyan on it and therefore you can ask for your money back? Clearly we would never say this, but rather we would say you were koneh the candle with the money by using the candle. So too by the Ner Chanukah we would say the same sevara and the money creates a kinyan. So too by the Ner Chanukah we would say the same sevara and the money creates a kinyan. (Disclaimer: I don't fully understand his sevara so I won't elaborate on it. Suffice to say the lack of clarity in what I wrote is due to my own lack of clarity and no reflection on Rav Kaplan).

However, nireh li that one could argue that the whole question never even starts. Who says that the neshtateif b'perutah works b'toras kinyanim. Why can't we just say that Chazal were mesakein a new din where by giving a perutah you somehow are able to be yotzei the mitzva of Ner Chanukah. I know I haven't really explained what the mechanism is that allows you to do this but at the very least we can say it is a takanas chazal. The main point is that who says it has anything to do with kinyan kesef.