You were quoting sources referencing irrelevant information from the Abassid Period. That is irrelevant. What's relevant is the hadith from Abu Darwood, who, in quoting sources from the Life of Muhammad, came up with the 2.5% figure. A legitimate, well established hadith. Your rant on the chronological history of the Zakat is irrelevant when looking at the Rashidun period. The Rashidun Caliphs, such as Abu Bakr, had a much more hands-on approach to Islamic jurisprudence than late Abassid Caliphs. When the capital was moved to Baghdad, the ruling Caliphs were often more disinterested than the Rashiduan Caliphs. Abu Bakr knew and lived with Muhammad, and would have known his hadiths, and the rules he established.