Such policies shall provide for a premium or premium deposit payable in cash, and in no event shall the holder of any such policy be liable for a greater amount than the premium or premium deposit expressed in the policy. Any insurance company organized and existing as a mutual fire insurance company of the state of Kansas shall have the authority and right to accept notes for premium on farm property. The unearned premium reserve on policies written on the installment note plan shall be determined by the amount of cash collected in excess of the percentage earned on policies written for a cash premium, and in no case shall the cash collected on such installment note policies be less than the percentage earned on policies written for a cash premium.