When you make a card transaction, the amount initially shown uses an indicative wholesale rate from Mastercard.
The final rate is confirmed when the merchant completes the transaction, usually within 1 to 2 days, though it can take up to 8 days.
Your split will not change. The split amount stays fixed at the time you create it.
For example, if you split a pending $100 transaction, the split amount remains $100, even if the final settlement rate differs.
To learn more about how exchange rates update, refer to this FAQ here.