Wash. Rev. Code § 43.09.2851
Except as otherwise provided by law, amounts charged by a county, city, or other municipal or quasi municipal corporation for providing services or furnishing materials to or for another fund within the same county, city, or other municipal or quasi municipal corporation pursuant to RCW 43.09.210 or other law shall be repaid and credited to the fund or appropriation against which the expenditure originally was charged. Amounts representing a return of expenditures from an appropriation shall be considered as returned loans of services or goods, supplies, or other materials furnished and may be expended as part of the original appropriation to which they belong, without further or additional appropriation.
Except as otherwise provided by law, this section shall not apply to the furnishing of materials or services by one fund to another when other funds have been provided specifically for that purpose pursuant to law.
RCW 43.09.2851