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 remain