Idaho Code § 57-502
When such refunding bonds are so sold the money received therefor, if state bonds, shall be deposited with the state treasury, or if the bonds of any county, school district, city, village, highway district, good roads district, irrigation district or drainage district, with the treasurer of such county, school district, city, village, highway district, good roads district, irrigation district or drainage district, such money so received shall become a special fund, and shall not be used for any purpose other than the payment and retirement of the outstanding bond issue so refunded; provided, that any balance left over after said bond issue is entirely retired, paid and canceled, may be transferred to the bond interest and sinking fund of the state, county, school district, city, village, highway district, good roads district, irrigation district or drainage district.
Idaho Code § 57-502