Provided also, that when the heirs and successors of an heir of entail, in any other than the entailed estate, are sued for the money due on account of improvements made upon an entailed estate under the authority of this Act, they shall be discharged in all cases from such suits, upon making payment of one third part of the rents of the entailed estate which have come to the use of such heir of entail, or to the use of his said heirs or successors.