Any unused portion of the credit for qualified rehabilitated buildings investment authorized by section twenty-three-a of this article which may not be taken in the taxable year to which the credit applies shall qualify for carryback and carryforward treatment subject to the identical general provisions under §39, Title 26 of the United States Code, as amended: Provided, That the amount of such credit taken in a taxable year shall in no event exceed the tax liability due for the taxable year.