It is permissible to advance zakat for one or more nisabs after owning the nisab. It is allowed to pay zakat for several nisabs even if one only owns one nisab. For example, if someone owns (2500) dinars, which is the nisab, and pays zakat on (10000) dinars, this is permissible for him, and what he pays is for zakat that will be due in the future. If he owns (15000) dinars at the end of the year, he has paid zakat on (10000) dinars of it, and if he only owns (5000) dinars at the end of the year, he has paid zakat for the coming years on (5000) dinars. The reason for this permissibility is that the first nisab is the original cause, and anything beyond it is dependent. Thus, if he owns more after the payment, what he has paid previously suffices as long as he owns the nisab. However, if he does not own a nisab at all, the payment is not valid, as explained in Al-Wiqayah, p. 217, and Umdat al-Ri'ayah, 1: 284, and Tabyin al-Haqaiq, 1: 275-276; it was narrated from Ali (may Allah be pleased with him) that "Al-Abbas (may Allah be pleased with him) asked the Messenger of Allah (peace be upon him) about advancing charity before it is due, and he permitted that," in Sunan Al-Darimi 1: 470, Al-Muntaka 1: 98, Sahih Ibn Khuzaymah 4: 48, and Al-Mustadrak 3: 375.