When exporting an invoice to Quickbooks I get a multicurrency error, why?

Question

I need to export an invoice from Forecast to Quickbooks but I get the following error. Why?

Answer

This error prompts when there is a different currency assigned to the client profile in Quickbooks than used in the Forecast invoice.

To resolve, update the currency assigned to the client in Quickbooks to match the currency on the Forecast invoice, then export again.