Whenever there shall be any money belonging to any levee district set apart as a sinking fund for the payment of bonds, the board of directors of such district shall loan the same for the highest obtainable interest, not exceeding eight nor less than six percent per annum, and shall cause the same to be secured by mortgage or deed of trust on real estate, free from all liens and encumbrances, within the county, of the value of double the amount of the loan; and any other funds of said levee district may be deposited at the highest obtainable interest in some bank in the county wherein said district or any part thereof is situated, in the same manner as county school funds are deposited; provided, no such loan or deposit shall be made until the same shall be approved by the county commission of the county in which said district may be situated.