The board of trustees shall have the power to defray the expenses of any improvement made by it in the execution of the powers hereby granted, by special assessment, by general taxation, or partly by special assessment and partly by general taxation, as it may determine by order. It shall constitute no defense to any tax or special assessment that the improvement, for which the same is imposed, lies partly outside of the limits of such district. No property shall be specially assessed more than it is benefited by the improvement for which the assessment is levied; Provided, that a deferred assessment, not exceeding the pro rata cost of the construction of sanitary sewers, may be made and collected solely in the form of a connection charge, if and when property not specially benefited is later connected to such sewer. The amount of such charge shall be fixed by the board of trustees at the same time such special assessments are made. Such deferred assessments shall not constitute a lien. The board of trustees shall make a detailed report of said deferred assessments and file such report with the county clerk in the county where the property is located.