The city or town councils of the various cities and towns may provide, by ordinance or through collective bargaining, for the retirement of the personnel of their police and fire departments who have been on leave of absence from their employment due to sickness contracted or injuries sustained in the performance of their duties; provided, that no ordinance is contrary to any pension cost of living increase or escalator clause in a collective bargaining agreement, and provided, further, that no ordinance provides for a disability retirement allowance of less than sixty-six and two-thirds percent (662/3%) of a retiree’s annual salary at the time of retirement nor more than one hundred percent (100%) of a retiree’s annual salary.