Must the difference in the zakat amount be paid if it was sent early in a different currency and the exchange rate changed by the zakat due date?

It is permissible to advance Zakat (definition) for one or two years if the wealth reaches the minimum threshold (nisab (definition)), and this is better than delaying it. If you pay the full Zakat before the year (hawl) is due, and your obligation is fulfilled, then a subsequent decrease in currency value will not harm you. It is permissible to transfer Zakat to another country if there is a need for it. The cost of transfer is borne by the owner of the wealth.