All assessments and tolls authorized under this act shall be paid in legal tender of the United States except that assessments and tolls levied for the expense fund of the district may be paid with district warrants issued in payment for labor hired by the district, at par without interest drawn on the expense fund in the year in which the assessment to be paid thereby is payable, or in the preceding year, and such warrants shall be so accepted notwithstanding their serial numbers or their order of issue as to then outstanding warrants: PROVIDED, HOWEVER, That in no case shall the county treasurer be authorized to pay any cash difference to the holders of any warrant so offered in payment of such assessments and in the event such warrant shall exceed the amount so applied on assessments, the county treasurer shall issue to the holder thereof a certificate directing the county auditor to issue to such holder a district warrant on the same fund, bearing date on which such lieu warrant is issued, for the difference between the face or par amount of the warrant received by the treasurer, without interest, and the amount credited on said assessment. Upon the surrender of such lieu warrant certificate the county auditor shall be authorized to issue and deliver such lieu warrant.