A purchase invoice in dollars or pounds stays in that currency in Kasklaar, and that is how it goes to your accounting system too: the amount as it is on the invoice, with the exchange rate alongside. Before you can approve, the currency has to be enabled and there has to be an exchange rate.
In the list and at the top of the invoice, the amount is in the invoice's currency. Once there is a rate, it appears under Invoice details at Exchange rate, and in the Payment block with the original and the converted amount.
From the payment. If your agents matched the bank transaction you paid the invoice with, they take the rate from it. If no payment is matched yet but it is on your bank statement, match it with Find match in the Payment block. Then you do not have to enter a rate yourself.
Without a payment, the Payment block shows the red notice "An exchange rate is needed to convert from USD to EUR".
To change the rate later, click Edit in the Payment block and then Edit exchange rate.
If the currency is not enabled yet, the Payment block shows a red notice with the button Configure currency or Enable currency. Click it and confirm. Kasklaar enables the currency for this administration and in your accounting system, and the notice disappears.
You can enable currencies in advance under Settings, Administration, Details, at Additional currencies. That only enables them in Kasklaar, not in your accounting system.