I made out an invoice to a customer in GBP, but he has paid me in EUR (on our EURO bank account). Now, QB does not "see" the invoice.
1. if you have multi-currency enabled, delete the GBP invoice, create a new EURO customer profile for the customer, and create a EURO invoice
2. Calculate the GBP amount of the EURO payment based on the rate on the payment date, and post the payment of the invoice amount to close it, with the difference to a Exchange Gain or Loss account