Upon retirement for duty disability, prior to becoming eligible for service retirement, as provided by Section 21-29-35, a member shall receive a duty disability retirement allowance, payable monthly, in a sum equal to fifty percent (50%) of his monthly salary from the employer at the time when the injury was sustained or the sickness occurred. This amount shall be paid to the member throughout such total disability for service. If the municipality pays a salary to such member in an amount equal to or in excess of the amount to be paid under this section, said member shall not be entitled to any payments under this section for the period of time during which such salary is paid.