then any benefits under subchapter II that were regularly due in such month or months, or supplemental security income benefits for such month or months, which are due but have not been paid to such individual or eligible spouse shall be reduced by an amount equal to so much of the supplemental security income benefits, whether or not paid retroactively, as would not have been paid or would not be paid with respect to such individual or spouse if he had received such benefits under subchapter II in the month or months in which they were regularly due. A benefit under subchapter II shall not be reduced pursuant to the preceding sentence to the extent that any amount of such benefit would not otherwise be available for payment in full of the maximum fee which may be recovered from such benefit by an attorney pursuant to subsection (a)(4) or (b) of section 406 of this title.