Short Answer

Since the amount to be paid was measured in food during the time of the Prophet, technically, the value would be calculated according to the destination region. However, it is more pious to calculate it according to the region in which you earned the money, since normally this charity should be given locally where the money was earned. So if Mustafa lives in America and would have paid the valuation for dates at $25, he should not pay only $5 because he sends it to Afghanistan and can get dates cheaper there.

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Shaykh Mustafa Umar