The Yerushalmi writes that neither issurei hana’ah d’oraysa or derabbanan can be used for kiddushei kesef. A shtar kiddushin, however, can be written on issurei hana’ah derabbanan but not issurei hana’ah d’oraysa. If a shtar must be written on something which has value, why are issurei hana’ah derabbanan acceptable? Why is there a difference between shtar and kesef?
As we learned in previous posts (see posts labeled KDS), R’ Elchanan suggested that there are two possible reasons why issurei hana’ah lack value: 1) metziyus - practically these items are worthless; 2) din – the halacha prohibits using these items. The difference between the two is with respect to issurei hana’ah derabbanan, which practically are off-limits, but on a d’oraysa level are halachically permissible to use.
With this distinction R’ Elchanan explains the Yerushalmi. Kiddushei kesef requires something that has real-world value, to the exclusion of even issurei hana’ah derabbanan. A shtar, however, does not have to be written on something that has value; it has to be written on something permissible to use. Since issurei hana’ah derabbanan are permitted to be used from a d’oraysa perspective, they are not disqualified.