Section 91 of Chapter 32 as so appearing, is hereby amended by striking out the first sentence in subsection (c) and inserting in place thereof the following sentence:

Each person referred to in paragraph (b) shall certify to his employer, the treasurer or other person responsible for the payment of the compensation for the position in which he is to be employed and to the retirement board from which the person retired, the number of days or hours which he has been employed in any such calendar year and the amount of earnings therefrom, and if the number of hours exceeds nine hundred and sixty, in the aggregate, he shall not be employed, or if the earnings therefrom exceed the amount allowable under paragraph (b), he shall return to the appropriate treasurer or other person responsible for the payment of compensation all such earnings as are in excess of said allowable amount. The amount of any excess not so returned may be recovered in an action of contract by the appropriate treasurer or other person responsible for the payment of the compensation of any such person by the retirement board from which the person retired.