Thanks for getting in touch with the Community about this, LittleGuy.
An exchange gain/loss expense occurs when selling or purchasing goods and/or services in a foreign currency and that currency fluctuates. Since this is inevitable, there isn't a way to avoid it. QuickBooks calculates gain and loss according to your exchange rate field on invoices. For example, when an invoice has a rate of 1:1.5 and a payment is recorded against it at 1:2.0, there will be an exchange gain/loss based on the difference in rates.
In regard to your second question, you're correct. Invoices can be edited anytime. You can additionally adjust the exchange difference from your Receive Payment screen when recording payments. Afterwards, you can check your Exchange Gain/Loss account by going to Accounting, then Chart of Accounts to see where it stands.
I've included a few detailed resources about working with multicurrency that may come in handy moving forward:
I'll be here to help if there's any questions. Have a great day!