A cause or operative legal ground supplies the basis to which a legal ruling is attached; an impediment prevents that basis or its result from operating in a specified situation.
For zakat, the qualifying property and the completion of a year are related to obligation. If the qualifying basis disappears during the year, the issue is completion of the operative cause rather than merely delayed payment. By contrast, a buyer's option upon seeing the goods concerns whether an already concluded sale remains binding.
Not every impediment should be collapsed into the absence of a condition. The fourfold classification identifies different legal effects and stages.