To the end that the principal of and interest on such refunding bonds may be further secured and thereby a more favorable sale or exchange effected, the resolution authorizing such refunding bonds may contain provisions which shall be a part of the contract with the holders of such refunding bonds as to the establishment of, and the making of appropriations for, reserve or sinking funds and the amount, source, securing, regulation and disposition thereof.
N.J.S. § App.A:4-68