RCW 35.21.650

All moneys, assessments and taxes belonging to or collected for the use of any city or town, including any amounts representing estimates for future assessments and taxes, may be deposited by any taxpayer prior to the due date thereof with the treasurer or other legal depository for the benefit of the funds to which they belong to be credited against any future tax or assessment that may be levied or become due from the taxpayer: PROVIDED, That the taxpayer may with the concurrence of the treasurer designate a particular fund of such city or town against which such prepayment of tax or assessment is made.