The city treasurer shall, within 1 week after the time specified and directed in the warrants annexed to said several December tax rolls, pay to the county treasurer the sums required in said warrants to be so paid, either in delinquent taxes on lands, or in funds then receivable by law, and all lands upon which any unpaid tax shall be returned shall be sold therefor the same as lands returned for delinquent taxes by township treasurers.