The Nisab of Zakah in US dollars

Q 1: 1- What does the Nisab (the minimum amount on which Zakah is due) equal in US dollars? It is stated in the book of Fiqh Al-Sunnah that the Nisab for gold is twenty dinars. However, we do not know what a dinar equals in Nigeria. However, we can know what a dinar equals in Nigerian Naira if we can know how much it equals in US dollars.2- How will a person whose money has never reached the Nisab and thus has never paid Zakah (obligatory charity) be judged on the Day of Judgment in this respect?3- A man owns the Nisab but he spends most, or all, of it before the passage of the Hawl (one lunar year calculated from the time a property reaches the minimum amount upon which Zakah is due). This happens every year, and thus the Hawl never passes after owning the Nisab - Allah knows best. Does he be exempted from Zakah?4- A man is never expected to own the Nisab unless Allah so wills. However, once he owns even a paltry sum of money, even if five Saudi Riyals or US dollars only, he pays 2.5% of it intending Zakah. Is there something wrong with doing so? (Part No. 8; Page No. 219

A: The Nisab for gold is twenty Mithqals (1 Mithqal = 4.25 grams) which equal twenty Islamic Dinars. Moreover, the Nisab for silver is 140 Mithqals which equals 200 Islamic Dirhams or any equal amount of banknotes, such as US Dollar, Saudi Riyals or any other currency. As for one who has never owned the Nisab, he is not liable for Zakah. Likewise, one who owns a Nisab and spends it before the passage of Hawl is not liable for Zakah. Furthermore, a poor person who collects as much Zakah money as may reach the Nisab is liable for Zakah after the passage of Hawl.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.