Each person who shall become an employe of the city after having been employed by the city in the bureau of fire or the bureau of police, and who shall desire to have such previous service counted for eligibility to receive a pension under the provisions of section 3 of this act,¹ shall be required to pay to the fund an amount equal to five per centum of his or her total salary or wages theretofore received by him or her, but in no event contributing more than twenty-five dollars ($25) per month for each previous month of service for which he receives credit as an employe of the city in the bureau of fire or the bureau of police with interest at the rate earned by the fund during the period of such prior employment. Full payment of such amount shall be a condition precedent to the member receiving credit for all or any part of said period of previous service for eligibility to receive a pension. Such amount shall be collected from the monthly salary or wages of the member over the period of two years in the manner provided in section 5 of this act,² or in such manner and period as the board of managers may determine. If, however, any such member shall be injured while in the actual performance of duty before he shall have made such full contribution for past service, so long as the disability continues, he or she shall be eligible to pension under this act, but any amount which he or she shall not yet have paid to the fund as a contribution for past services under this section at the date of his or her injury shall be paid, if the board, in its discretion, shall so determine, in such monthly payments as the board may determine, which amounts shall be deducted from his or her pension as and when monthly payments thereof shall be made.