Section 8. An employer who has contributed to an insurance fund created and maintained for the mutual purpose of indemnifying an employee for personal injuries for which compensation may be recovered under section one or to any relief society formed under sections eighty-six, eighty-seven and eighty-eight of chapter one hundred and fifty-nine, or the corresponding provisions of earlier laws may prove in mitigation of the damages recoverable by an employee under said section one, such proportion of the pecuniary benefit received by such employee from any such fund or society on account of such contribution of said employer as the contribution of such employer to such fund or society bears to the whole contribution thereto.