In theory Discpunt for Purchase item should not even be pickable on a Sales Order.  Just like other item that are “For Purchase” it should be locked to only the purchase side of the house (procure to pay).  BUT I just tested this an of course this is a bug and doesn’t work correctly.  Seems the like distinction of “Discount for Purchase” versus “Discount for Sale” is determined by the first account when you first save it.  Then you can change the account but the subtype won’t change.  But then the subtype doesn’t acctually do anything.  I was able to add a “Discount for Purchase” to a Sales Order but that should have been blocked.  So I call that a bug.

So a workaround would be use the feature where you can put a saved search on a custom form to filter what items are pickable on that form.