Cannot set overall discount on sales order by API

curl --data 'operation=revise&sessionName='$sessionName'&element=%7B%22id%22:%226x1059807%22,%22hdnDiscountAmount%22:%2227.270%22%7D' https://${crmsite}/webservice.php

returns

{"success":true,"result":{"id":"6x1059807","hdnDiscountAmount":"0.00000000","conversion_rate":"1.000","salesorder_no":"SO212624" [...] }}

and inspection via the UI confirms that the overall discount remains at 0.0. It should show "hdnDiscountAmount":"27.270"