Second part of your question:
Zakah is applicable to the amount that is over and above the exempted amount (Nisab). If the income of that couple is below that Nisab, then there is no zakah applicable on that couple.
The couple’s Zakah on Produce (i.e. The return on savings):
Their capital / principal amount of investment is exempt from zakah (because it is considered as a tool of production which is exempt in case of produce), only zakah that they’re obligated to pay is on the amount of profit they receive from those savings; provided that profit is over and above the Nisab amount. The zakah of 10% would be applicable on their amount of return over and above the nisab.
We consider the return on savings a form of produce that’s why we linked it to category of produce.
For nisab on produce, Govt. provided exemption amount can be used as provided in tax laws. In its absence, the pure nisab for produce should be used. i.e. amount equivalent to 653 kilograms of dates.
The couple’s Zakah on Year End Wealth:
And at year end, on the day fixed by the couple, any year end amount of their wealth, other than exempted items and amount below the nisab for wealth, zakat would be 2.5% on those savings. (the principal amount of savings would still be exempt and not included in the wealth of year end calculation because it is not a part of their immediate wealth as it is locked in as an investment principal and not readily accessible).
Nisab for year end calculation of wealth is amount equivalent to 642 grams of silver.
Here is law of zakah summarized:
You can calculate Zakah in the following manner:
1. Fix any one day of the year for the calculation of Zakah. On all subsequent years, Zakah should be calculated on this day.
2. Subtract the value of 642 grams of silver from your total wealth on this particular day which includes value of your jewelry not for personal use, value of other assets which are not of personal use (your personal house, jewelry for personal use and car are examples of assets in personal use).[1] & [2]
3. 2 1/2 % of this amount is the Zakah to which you are liable. In case you are a salaried person you should also pay 10 %[3] & [4] of your salary every time you receive it. You can deduct the amount the government has fixed as exemption to salaried persons and then pay the said percentage of the remaining amount as Zakah.
Remember, if it is not possible to pay your Zakah money in one go you can pay in installments as well.
4. Since Zakah is the only tax which a government can impose on its citizens, you can deduct other taxes that you have paid from your Zakah amount. If the balance is negative you do not have to pay Zakah.
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[1] Nothing except the tools of production, personal items of daily use and a fixed quantity called nisab are exempt from zakah. It shall be collected annually on wealth of all sorts, livestock of all types and produce of all forms of every Muslim citizen who is liable to it.
[2] This nisab is as follows:
(a) Wealth: 5 uqiyahs / 642 grams of silver
(b) Produce: 5 wasaqs / 653 kilograms of dates
(c) Livestock: 5 camels, 30 cows, 40 goats
[3] If the basis of the Sunnah established in Zakah is taken into consideration, production of all forms based on various skills must be classified as produce and not as wealth; therefore, its rates and nisab should be those specified by the Prophet (sws) for land produce.
[4] Produce: (i) 5% : on all items which are produced by the interaction of both labor and capital, (ii) 10% on items which are produced such that the basic factor in producing them is either labor or capital and (iii) 20% in items which are produced neither as a result of capital nor labor but actually are a gift of God.