(b) In any assessing unit in which there has been a change in level of assessment as provided in title two of article twelve of the real property tax law, the commissioner of taxation and finance shall certify the change in level of assessment factor in the manner provided in title two of article twelve of the real property tax law. The term "assessing unit" as used in this subdivision means a city, town, village or county having a county department of assessment with the power to assess real property. 5. The tax exemptions specified in subdivisions three and four of this section shall not operate for a period of more than fifty years, commencing in each instance from the date on which the benefits of such exemption first become available and effective, except that in the case of a federal project the tax exemption may operate for a period not to exceed sixty years, and except that in the case of a project, or part thereof, leased from a housing company or in the case of a federal or municipal project consisting of land and one or more multiple dwellings leased by an authority for not less than ten years the tax exemption may operate for a period not exceeding the term of such lease. 6. Upon the completion of the sixty-year period provided in subdivision five of this section applicable to federal projects, each authority that was eligible for the tax exemption specified in subdivisions three and four of this section shall be exempt from all local and municipal taxes imposed by state and local governments for sixty years after the expiration of the initial exemption period. Upon the completion of the fifty-year period provided in subdivision five of this section applicable to state and municipal projects, each authority that received the tax exemption specified in subdivision three or four of this section may, on the expiration of the tax exemption period, be granted an additional tax exemption period of up to fifty years. A municipality may (a) fix a sum which shall be paid to it annually by the authority in respect of each project; or (b) agree that the authority shall not pay or be liable to pay any sum whatsoever in respect of a project or projects for any year or years; or (c) agree with an authority or government upon the sum to be paid by the authority for any year or years in respect of a project or projects, or accept or agree to accept a fixed sum or other consideration in lieu of such payment; provided, however, that the sum fixed, or agreed to be paid by the authority, for any year shall in no case exceed the sum last levied as an annual tax upon the property included in such project prior to the time of its acquisition by the authority or, in the case of a federal project, such greater amount as the federal government may permit. With respect to a federal project located within a school district, an authority may make payments to such district, and the authority and the district may enter into agreements with respect to such payments. Such payments shall not exceed the amounts permitted for such purposes by the federal government. The power granted by this subdivision shall be in addition to any other power granted in this section or any other provision of law.