Section 11. The municipality may raise and appropriate or may borrow, or may agree with the corporation or with the federal government or the commonwealth to raise and appropriate or to borrow, in aid of the corporation, such sums as may be necessary to carry out the purposes and powers of the corporation including defraying part of the development, acquisition and operating costs of any project. Indebtedness of the municipality authorized under this section shall be outside the limit of indebtedness prescribed in section ten of chapter forty-four and shall be payable within twenty years and otherwise subject to sections sixteen to twenty-seven, inclusive, of said chapter forty-four; provided, however, that the total amount of indebtedness of the municipality, outstanding at any one time under this section and clauses one, two, and four of section twenty of chapter one hundred twenty-one B shall not exceed five per cent of the municipality equalized valuation as defined in section one of said chapter forty-four.