Ibn Tawus narrates from his father that Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) was asked about ambergris: is there zakah on it? He said: If anything is due on it, then the fifth (khumus) is due on it.
These are the words of the narration of Imam al-Shafi'i (may Allah have mercy on him). In the narration of Ibn Shayban: Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) was asked about ambergris: is there zakah on it? Then he mentioned the rest. So in this narration Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) made his statement conditional, while in the first narration he stated decisively that there is no zakah on it; and the decisive statement takes precedence.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الزكاة / 7595