Nothing contained in sections 8-815 to 8-826 shall be construed as preventing a bank from purchasing or discounting from established business concerns any commercial, negotiable or installment paper, or as preventing any such bank from accepting from, or requiring such persons selling or offering to discount such instruments to execute, contracts guaranteeing the ultimate collection of all of such items so sold or discounted or requiring such persons to assume the burden of making collections of the individual items so sold as agent of the bank.