Adding a payment to an invoice is only available in USD

The box is greyed out and is prefilled with USD. I have four currencies active and the company, the customer and the invoice are all set to NZD and NZD is the default.

What's even more messed up is that adding a $10 payment is processed as $8. That's likely due to the exchange rate set in currencies BUT on the main dashboard it shows an $8 profit but has $10 in the default cash account.

Edit: OK, I figured out that the default 'cash' account that it went into is in USD, which explains that. Perhaps could be clearer, or when changing the default currency of the company it updates the accounts (or at least prompts you to).

I would advise to check the documentation for currencies:

https://akaunting.com/docs/user-manual/settings/currencies

You're also welcome to contribute to the documentation.