The cost and expense attending the sale and assignment of a special assessment lien by a municipality, not exceeding 10% of the amount of the lien, shall be assessed as costs and shall be paid by the assignee. However, no lien created after September 1, 1949 shall be sold or assigned by a municipality as long as any obligation of any kind secured by such lien remains outstanding and unpaid.
65 ILCS 5/9-2-71