kiddushin using chameitz d'rabbanan

[In case you missed last week's post, I've started a new series focussing on learning through R' Elchanan's 'Kuntres Divrei Sofrim'. You can sort by the label KDS on the post to see others in the series.]

Why does the KS”M use such elaborate reasoning to explain the Rambam instead of taking the more direct approach R’ Elchanan suggests? Furthermore, why would the Rambam use an example like chameitz when the KS”M’s approach would work with any issur hana’ah derabbanan?

The first question becomes a little weaker if one takes the KS”M in the context of the perek (Mamrim 4) as a whole, where the Rambam lists many other examples of halachos that fall under the zakein mamrei umbrella because they can have ramifications for questions of chalos kiddushin. For example, the Rambam writes:

וכן אם נחלקו בלקט שכחה ופיאה, אם זה לעניים או לבעל הבית--הרי זה חייב: שהרי לדברי האומר לבעל הבית--הרי זה גזל ביד העני, ואם קידש בו אישה אינה מקודשת

With respect to the second point, R’ Elchanan points us to the Rambam in Ishus 5:1

המקדש בדבר שהוא אסור בהנאה--כגון חמץ בפסח או בשר בחלב, וכיוצא בהן משאר איסורי הנאה--אינה מקודשת; ואפילו היה אסור בהנאה מדבריהם, כגון חמץ בשעה שישית מיום ארבעה עשר--אינה מקודשת.

Notice that when the Rambam speaks of issurei hana’ah d’orayasa, he uses two examples: basar b’chalav and chameitz. When he speaks of issurei hana’ah derabbanan, he uses only the chameitz example. Why? The KS”M writes that chameitz is the single case of issurei hana'ah derabbanan which compromise chalos kiddushin. Unlike other issurim which are a new cheftza shel issur derabbanan, chameitz derabbanan meets the Biblical definition of what chameitz is, and is simply an extension of the time period during which chameitz is prohibited.

The KS"M concludes that using a type of chameitz which is only defined as such m'derabbanan during this extended time period to effect kiddushin should not pose a problem. More on this to come bl"n.