kiddushin using chameitz derabbanan

Chameitz derabbanan includes two categories of prohibitions: 1) items not defined Biblically as chameitz, but which Chazal prohibit; 2) items which the Torah defines as chameitz eaten during the early hours of Erev Pesach when the issur applies only mederabbanan. The Kesef Mishne (Ishus 5:1) holds (as previously discussed) that chameitz is the only issur hana’ah derabbanan which cannot be used for kiddushin. The KS”M infers from the Rambam that this is true only in the two scenarios above. However, if one combines the derabbanan aspects of both scenarios, i.e. using for kiddushin an item classified as chameitz mederabbanan during the time period where chameitz is prohibited only mederabbanan, the kiddushin is chal. R’ Elchanan asks: If chameitz derabbanan is categorically excluded from use for kiddushin, why should compounding multiple derabbanans make any difference?

With respect to chameitz derabbanan, Rashi (Pesachin 7a d”h afilu b’chiti) explains that Chazal dissolved the kiddushin based on the principle of “afk’inhu rabbanan l’kiddushin”. The implication of Rashi is that the kiddushin is valid min haTorah. R’ Elchanan notes that a far simpler explanation of why the kiddushin is invalid is possible. For kiddushin to be valid the husband must give an item valued at at least a perutah to his kallah. Since issurei hana’ah – whether d’oraysa or derabbanan – have no value, they cannot be used for kiddushin. The kiddushin should therefore have no validity min haTorah. Why does Rashi need to invoke the principle of “afk’inu l’kiddushin” instead of using this simpler explanation?

R' Elchanan answers both questions with one principle... stay tuned.