If the Nisab is reached on the tenth of Sha'ban, then it decreases on the same day of the following year, and then it reaches the Nisab again in the month that follows, does one pay the Zakat immediately, or does one wait for a new Hawl (lunar year)?

One of the conditions for the obligation of Zakat is that the nisab (minimum taxable amount) must not decrease during the hawl (one lunar year). If it decreases, the hawl is interrupted and restarts anew when the nisab is complete again. So, if the hawl for one's wealth started on the tenth of Sha'ban, and the wealth decreased below the nisab on the same day the following year, then no Zakat is due on it. However, if the nisab became complete in Ramadan, the hawl would begin from Ramadan, and Zakat would become obligatory in the following Ramadan, provided the wealth did not fall below the nisab during the hawl.