What is the ruling concerning the destruction of the surplus portion after the Zakat obligation has been established?

Primary text

If the surplus portion exceeding the Nisab is destroyed before the owner is able to render the Zakat, no part of the established Zakat obligation is remitted, because the destroyed wealth was not the subject of the Zakat obligation. If, however, a portion of the actual Nisab is destroyed, the Zakat is reduced proportionally; for example, if five of the Nisab are destroyed, one-fifth of the due Zakat is remitted because the destruction is calculated against the wealth upon which Zakat was due.