When you add or edit a client, there is an option to determine the currency that they get billed in for fulfilment activity. To do this:
Click Clients then Overview.
Find the client.
Next, click Actions then Edit.
Click the Defaults tab.
Use the CurrencyId drop-down and select the required currency.
By default, this will be set as GBP £.
Finally, click Update.
Invoice data will now be displayed in the desired currency.