No franchise granted by the city shall be leased, assigned or otherwise alienated without the express consent of the council entered upon its minutes, and no dealings with a lessee or assignee on the part of the city to require the performance of any act or payment of any compensation by the lessee or assignee shall be deemed to operate as such consent; provided, that nothing therein shall be construed to prevent the owner of such franchise from including it in a mortgage or trust deed executed for the purpose of obtaining money for corporate objects.