The zakatable-minimum is $2000. One possess $1000 of cash, $1500 of gold and silver, and $2500 in trade goods then one’s total zakatable assets amount to $5000 (supposing one has no money lent out, or stocks). However, one also has a debt of $500 and immediate monthly expense amounts to $500 also, which will be subtracted. Thus, the total zakatable wealth is $4000. This is above the zakatable-minimum, so the obligation to pay Zakat will commence.
If one possessed this amount on the 1st of Rabi‘ al-Awwal then this is when the Zakat year starts. If then, for example, one possesses $2500 on the 1nd of Rabi‘ al-Awwal of the next year, one will have to pay 2.5% of $2500. Thus, what is taken into consideration is the amount one possesses above the zakatable-minimum at the end of one’s Zakat year. This is the amount Zakat is due upon.
Fluctuations during the middle of the year are of no consequence, unless one’s zakatable amount reaches zero. Only the beginning and end of the year are taken into account. One therefore has to possess a zakatable-minimal both at the beginning and at the end of one’s Zakat year.