Should any gift or donation to said department be in the form of real property, it may be leased, rented or sold in the discretion of said board of trustees, but the sum received as rent or the amount received as the purchase price, in the event of sale, must be deposited to the credit of said endowment fund, and such sum shall remain intact as a part of the principal amount of such endowment fund, and the interest received from the investment thereof shall be paid in the same manner as provided in this article for the payment of interest on other moneys deposited to the credit of said endowment fund.