Question: If the obligation of the b'nei ha-ir to give again (as stated by Resh Lakish) is a takanah, as the GR"A states, why is the shevuah a shevuat takanah and not a shevuah d'orayta? If the chachamim imposed upon the b'nei ha-ir to pay again, the obligation is a real obligation upon them (presumably based upon the principle of hefker bet din hefker). If it is a real obligation upon them, then the shevuah concerns a monetary obligation of of the b'nei ha-ir and does not involve an obligation to hekdesh. It therefore seems the shevuah should be a shevuah d'orayta.
If the failure of the B'nei ha'Ir to fulfill their obligation was due to the Shaliach's failure to fulfill his mission, and thus they would be required to replace the money as a result of his failure, then it seems that your assertion would be correct, that the Shevu'ah should be a Shevu'ah d'Oraisa.
However, the Gemara here is of the opinion that as soon as an object is given to Hekdesh, it is considerd to be the property of Gavoha. Therefore, the B'nei ha'Ir have actually fulfilled their obligation, and are only required to pay again due to a rabbinic enactment; they are not required to replace the money that the Shali'ach lost, but they pay again to ensure that they will be careful in their guarding of Hekdesh in the future, as explained by the Vilna Ga'on. This obligation is not considered to have been caused by the Shali'ach but by the Rabanan, and therefore the Shali'ach's Shevu'ah is only a Shevu'as Takanah.
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-D. Schloss