Cancel Save Workflow Query

Hi All,

    I have a query regarding normal workflows and cancel save work flows. Are they different in the first place? I have a requirement where I am using a cancel save workflow (before modified record saved)

  The condition is as below

   1.The opportunity in particular sales stage and one field being not null should be updated only webservices and not by user. I used the below expression and it worked

            Sales= 'XXX' AND HID IS NOT NULL AND NOT(ExcludeChannel("Webservices"))

The problem came when my requirement was to edit the opportunity through webservices only when the above condition was met. On other ocassions I was not able to do it. To get web services working at all times I changed the above condition as below but it allowed user to edit my opportunity.