Levies may be made on lands mortgaged as on lands not mortgaged, and the amount due on the mortgage may be deducted by the appraisers from their estimated value, and stated in their return. If the full amount due was not deducted, or if the levy was made in the usual form, and it is ascertained that there was a mortgage on the premises, not including other real estate, and not known to the creditor at the time of the levy, it shall nevertheless be valid, and the creditor may recover of the debtor the amount which should have been and was not deducted, or the amount due on such mortgage.