How is the Zakat calculated for a sum of money, a portion of which was converted to another currency, then a portion of the second currency was re-converted to the original currency, with a differing exchange rate?

The criterion for the zakat of current money is to compare it to unwrought gold or silver. If the amount equals or exceeds the zakat threshold (85 grams of gold), and a year has passed on it, then zakat is obligatory on it. However, if it is less than that, then no zakat is due. Zakat is to be paid from the money itself.