Answer
(Fatwa: 518/518=M/1429)
وسببہ أي سبب افتراضھا ملک نصاب حولي تام الخ
It means that the reason of Zakah is to own wealth equal to nisab. So, Zakah will be liable to be paid as long as the reason i.e. nisab exists. Hence, if you own amount equal to nisab in the beginning and end of the year then you have to pay the Zakah every year; since owing nisab will require that Zakah is paid every year, as this is the rule for salah, sawm etc.
Allah (Subhana Wa Ta’ala) knows Best
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