The grantee of a franchise may not sell, assign, sublet or allow another to use the same, unless the Council gives its consent. Nothing in this section shall limit the right of the grantee of any public utility franchise to mortgage its property or franchise, nor shall restrict the rights of the purchaser, upon foreclosure sale, to operate the same, except that such mortgagee or purchaser shall be subject to the terms of the franchise and provisions of this article.