(b)In cases falling under sub section (5) (a), where the property has been leased back to the mortgagor by the mortgagee, nothing contained in that sub section shall affect the right of the mortgagee to recover any rents due to him under the lease for any period before the date on which the mortgage debt is deemed to have been wholly discharged by virtue of that sub section, if such rents have not become barred by limitation under any law for time being in force.