الجواب وباللہ التوفیق 

1. In principle, the nisab for discharging zakat is seven and a half tolah gold and fifty two and half tolah silver. The details of this calculation are described that if someone only owns gold, but does not own any silver, cash or other articles of trade, then in order for zakat to become wajib on this person, he should own seven and a half tolah of gold after deducting the amount needed for fulfilling his basic necessities. If he does not own seven and half tolah after the above mentioned deduction, then zakat will not become wajib upon him. Similarly, if the person only owns silver and no gold or cash and any other articles of trade, then in order for zakat to become wajib on this person, he should own fifty two and a half tolah of silver, after deducting the amount needed for fulfilling his basic necessities. If he does not own fifty two and a half tolah of silver after the above mentioned deduction, then zakat will not become wajib upon him. Otherwise, if someone owns a combination of these articles (i.e. gold, silver, cash, articles of trade etc.) and in itself each is less than the nisab of zakat, e.g. he owns ten tolah of silver, two tolah of gold, five thousand in cash, and none of these item of wealth individually reaches the Shar’i nisab then their combined value will be compared against the nisab of silver. If their combined value, after deducting the wealth needed for fulfilling the basic necessities and expenses of the household, reaches the value of fifty two and a half tolah silver, and a year has passed on this wealth, then zakat will have to be discharged as two and a half percent of the combined wealth. For example, two and a half rupees on a total of hundred rupees or two and a half thousand on one hundred thousand rupees.
2. At the time of discharging the zakat, the value of the gold, silver, articles of trade, investments in business, cash etc. will be combined and if the resulting amount is equal to or more than the amount of nisab, then two and a half percent of this combined value will be discharged as zakat.
3. It is incorrect to discharge zakat on the basis of estimate, it is possible that the estimated zakat is an overpayment and the payer will get reward for it, but if it is underpaid then it will be a sin. Therefore, zakat should be discharged after calculating and taking in account all forms of ones wealth.
4. If the wife is Sahib-e-Nisab (a person on whom discharging zakat is mandatory), then it will be mandatory upon the wife to discharge her zakat, and not the husband. However, if the husband discharges zakat on her behalf, then zakat will be considered as paid. If the husband is Sahib-e-Nisab, then paying his own zakat is mandatory upon him (other than what he paid on his wife's behalf).

’’ فی البدائع ماذکرمن وجوب الضم إذ الم یکن کل واحدمنہانصابابأن کان أقل فلوکان کل منہمانصاباتامابدون زیادۃ لایجب الضم بل ینبغی أن یؤدی من کل واحدزکوتہ فلوضم حتی یؤدی کلہ من الذہب أوالفضۃ فلابأس بہ عندناولکن یجب أن یکن التقویم بماہوأنفع للفقراء ‘‘ … ( الدرمع الرد : ۲ ؍ ۳۷ )

فاذا کانت مائتین وحال علیھا الحول ففیھا خمسۃ دراھم الخ ولیس فیما دون عشرین مثقالا من ذھب صدقۃ فاذا کانت عشرین مثقالا علینا نصف مثقال الخ وفی تبر الذھب والفضۃ علیھما وادا نیھما الزکوٰۃ (ہدایہ باب زکوٰۃ المال ج ۱ ص ۱۷۶ و ص ج۱ ص ۱۷۷)

فلیس فی دور السکی و ثیاب البدن واثاث المنزل ودواب الرکوب وعبید للخدمۃ وسلاح الا ستعمال زکوٰۃ فتاویٰ عالمگیری کتاب الزکوٰۃ ج۔۱ ص۱۷۲

فأ فاد ان التقویم انما یکون بالسکوک عملاً بالعرف مقوما بأ حد ھما ان یستویا ، فلو احدھما اروج تعین التقویم بہ ولو بلغ بأ حدھما نصابادون الأخر تعین ما یبلغ بہ ولو بلغ بأحد ھما نصا باوخمسا وبالآخر اقل قومہ بالأ نفع للفقیر سراج ، ربع عشز درمختارمع الشامی باب زکاۃ المال ج۔۲ ص۲۹۹۔

’’ فی البدائع ماذکرمن وجوب الضم إذ الم یکن کلواحدمنہانصابابأن کان أقل فلوکان کل منہمانصاباتامابدون زیادۃ لایجب الضم بل ینبغی أن یؤدی من کل واحدزکوتہ فلوضم حتی یؤدی کلہ من الذہب أوالفضۃ فلابأس بہ عندناولکن یجب أن یکن التقویم بماہوأنفع للفقراء ‘‘ … ( الدرمع الرد : ۲ ؍ ۳۷ )

فقط واللہ اعلم بالصواب