Nisab for production is not 642 grams of Silver. Following is the correct understanding regarding Nisab:
– Nisab for production / earnings is the minimum amount below which your state exempts tax or zakat on earnings. If your state does not provide a nisab then you can use the income tax exemption as Nisab and in the absence of all these options, standard nisab of 653 kg of Dates or equivalent value can be used.
– Nisab for assets/wealth is the minimum amount below which your state exempts tax or zakat on assets. If your state does not provide a nisab then you can use the standard nisab of 642 grams of silver or equivalent value.
Since salary belongs to the domain of earning / production, therefore, in the absence of any Nisab for Zakat given by the government, Ghamidi Sahab has done ijtihad that for the time being, it is appropriate to use the income tax exemption as Nisab for the salary/production.
However, there can be difference of opinion on whether Zakat percentage should be applied on amount only above Nisab threshold or on the whole amount as soon as it crosses the Nisab threshold. Ghamidi Sahab prefers the latter approach but accepts room for a difference of opinion in this regard. Please see these responses of Ghamidi Sahab in this regard:
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