Section 11. Cities, towns and districts may pay or provide for the payment of any debt at an earlier period than is required by this chapter or may appropriate money for the purchase and cancellation of outstanding indebtedness, and shall not refund any debt except as provided in section seventeen; and cities, towns and districts may, for the purpose of reducing the whole debt for the redemption of which sinking funds have been established by cities or towns prior to May twenty-eighth, nineteen hundred and thirteen, and by districts prior to March twentieth, nineteen hundred and fifteen, or of reducing the amount to be raised by taxation for such funds, add to any such sinking fund the excess of any appropriation over the amount required for the purpose thereof, or add to such funds any sums derived from taxation or from other sources not required by law to be expended otherwise.