Zakah becomes fardh when one has cash equal to the price of 612 gram silver and it becomes wajib to pay 2.5% zakah on completion of the year. So pay 2.5% of the cash which you have. You will have to pay its zakah though you have saved it for your future requirement until it is in the form of cash with you. . But when it is spent in any need then zakah shall no longer remain wajib.
Allah knows Best!
Darul Uloom Deoband