The engineer in charge of the work shall furnish the contractor a monthly statement estimating the amount of work done on each section and in each county. A duplicate copy of the statement shall be filed with the auditor of each county where the work is done. When the auditor files the statement, the auditor shall draw a warrant for the contractor or give the contractor an order directing the treasurer to deliver to the contractor improvement certificates or drainage bonds, as the case may be, in favor of the contractor for ninety percent of the amount due from the auditor’s county. Drainage warrants, bonds, or improvement certificates when so issued shall be in such amounts as the auditor determines, but shall not be in amounts in excess of five thousand dollars.