§ 121. Sinking fund. Unless other provision be made therefor in the contract with the municipality, the supervising agency, if it shall deem it feasible at any time, subject to the limitation contained in section one hundred seven of this article, may require a redevelopment company to provide out of earnings, after provision for distributions and interest, a sinking fund in an amount to be fixed by such supervising agency for the gradual retirement of the capital and income debenture certificates of such company. Such sinking fund may be used either for the purchase or retirement, from time to time, of capital or income debenture certificates at a price approved by the supervising agency not exceeding par value thereof with accrued and unpaid distributions or interest, or if it be not practicable to purchase or retire such capital or such income debenture certificates at a price so approved, the moneys in such sinking fund may be added to the surplus of such company. Any capital evidenced by shares or income debenture certificates purchased or retired out of such sinking fund shall be cancelled and shall not be reissued.