§ 157. Release of part of mortgaged premises. If the owner of mortgaged premises sell a part thereof, the comptroller, on application and with the consent of the mortgagor or such owner may release the part of the mortgaged premises sold from the lien of the mortgage. Such release, however, shall not be given unless a sum approved by the comptroller shall be first paid upon the mortgage and unless the part of the mortgaged premises remaining unsold, exclusive of buildings and prior liens, is worth double the residue of the mortgage debt. The comptroller shall execute such release in the usual form, which, when acknowledged, shall be recorded by the county clerk and a minute thereof made upon a margin of the mortgage.