Section 32. In a city or town which accepts this section the board of trustees of the public library or the city or town official possessing the appointive powers of such board may grant a leave of absence for study or research to any library staff member which would enable him to increase his professional ability, such leave to be for a period not exceeding one year at full or partial pay; provided, that prior to the granting of such leave said library staff member shall enter into a written agreement with said trustees or official that upon the termination of such leave he will return to service in the public library of such city or town for a period equal to twice the length of such leave and that, in default of completing such service, he will refund to the city or town an amount equal to such proportion of salary received by him while on leave as the amount of service not actually rendered as agreed bears to the whole amount of service agreed to be rendered.