A member who is or has been granted a leave of absence without pay to pursue a course of study which is connected with the work of the agency and which is for the purpose of increasing his proficiency in his position shall be entitled to credit towards retirement for the period of such leave if he makes his retirement contributions for each year of such leave in an amount equal to five per cent of his salary rate at the time he was granted leave, with interest at the rate of five per cent per year.