Is zakat due on all money if it reaches the nisab and a year has passed over it, with a new sum added to it, or is it only due on the money over which a year has passed?

Zakat is obligatory under two conditions: the wealth reaching the nisab (threshold) and the passing of a hawl (lunar year) over it. So, if the initial wealth reaches the nisab and a hawl passes over it, its zakat becomes obligatory. As for wealth added later, it is permissible to pay its zakat along with the initial wealth by expediting it, or to start a new hawl for it from the date of its possession. It is then zakatable after its hawl even if it does not individually reach the nisab, because it is combined with the initial wealth for reaching the nisab, but not for the hawl.