Section 6A. As soon as any such corporation determines to carry out a project approved by the housing board, such corporation and the city in which its project has been authorized, acting by its city manager or, if there is no such official, its mayor, or the town in which its project has been authorized, acting by its selectmen, shall contract for the carrying out of such project in accordance with the application, the provisions of this chapter, and the rules, regulations and standards prescribed by the housing board for such project. Such contract may provide that, without mutual consent, any subsequent amendment of any such provisions, rules, regulations and standards shall not affect the project. Nothing in section ten shall prevent such contract from further providing for such corporation to pay to the city or town with respect to one or more years such specific or ascertainable amount in addition to the excise prescribed by section ten as may have been stated in the application.

Any such contract may further provide that such corporation may elect as of the end of any fiscal tax year ending after the fifteen year as provided in section ten to shorten the period of extension of tax exemption as further provided by section ten by written notice given to the clerk of the city or town and to the housing board not less than thirty days prior to the date upon which the assessment of property is set during any municipal fiscal tax year provided that no such action shall be permitted hereunder unless all the amenities upon which the approval of the period of extension of tax exemption was based, have been established, as certified by the housing board.

Any contract or agreement executed pursuant to this chapter shall be filed with the housing board and the office of the clerk of the city or town in which the project is located and shall be made available to any person in accordance with the twenty-sixth clause of section four of chapter seven.

Except in the city of Boston and Springfield, any contract or agreement executed pursuant to this chapter shall be adopted and approved prior to execution in the manner set forth in section six. No contract or agreement under this section shall be approved unless the signatures of a majority of the assessors is affixed thereto.

All amounts payable, in addition to the excise prescribed by section ten, pursuant to a contract or agreement executed under this section shall be in lieu of taxes assessed and levied upon the corporation's real and personal property and shall be collectible as such against the corporation and its property in accordance with chapters fifty-nine and sixty.