The currency can't be changed
If you cannot change the currency on an invoice (or another document), it is because changing it after amounts exist would make the figures inconsistent. Here is the rule and what to do.
The rule
You can set or change a document's currency while it is a draft and before it has any lines. Once lines exist, the currency is fixed. You will see a message such as: "Cannot change invoice currency once invoice lines exist. ... remove all invoice lines first or create a new invoice."
For credit notes, the currency must match the invoice they relate to, and it is fixed once the credit note is issued.
What to do
- The document is still a draft: remove all its lines, change the currency, then re-add the lines. Or simply start a new document in the currency you want.
- The document is already issued or posted: its currency cannot change. If it is wrong, void it and create a new one in the correct currency.
Setting a customer's default currency instead
If you keep having to change the currency for the same customer, set their preferred currency on their customer record. New documents for that customer will then default to it automatically — see Working with foreign currency.