As the mortgage sought to be foreclosed in this case, was given to secure the purchase money on land sold by the mortgagees, with covenants of seizin and against incumbrances, and it turns out that there was at the time of the conveyance an outstanding mortgage which still remains an incumbrance upon the premises, that mortgage must be first removed before a decree for foreclosure and sale can in equity be ordered; or, so much of the proceeds of the sale as may be necessary for that purpose, must by the decree be directed to be applied to pay off and satisfy that incumbrance, and the amount so applied deducted from the mortgage debt due the complainant