56. If the owner of two or more properties mortgages them to one person and then sells one or more of the properties to another person, the buyer is in the absence of a contract to the contrary, entitled to have the mortgage-debt satisfied out of the property or properties not sold to him, so far as the same will extend, but not so as to prejudice the rights of the mortgagee or persons claiming under him or of any other person who has for consideration acquired an interest in any of the properties.