The terms of payment on the sale of university and school lands shall be 50 per centum of the purchase money to be paid at the time of the purchase, the balance of the principal at any time thereafter at the option of the purchaser, with interest at the rate of 7 per cent per annum, on the unpaid balance, payable on the first day of March, or within 60 days thereafter in each and every year at such place or places as shall be specified in the certificate of purchase: Provided, That before any of said lands shall be sold on part payment at the time of purchase, the commissioner of the state land office shall require the affidavits of at least 2 persons (accompanied by the certificate of the supervisor of the township in which such lands are situated as to the credulity [credibility] of such persons), that such lands are not valuable chiefly by or on account of timber thereon.