Currently, only the home currency sign will reflect in printing reports for the customer. Let's say your home currency is United States Dollar, then you can inform your customer that it is printed as dollar. However, you can export your report in Excel file to customize the sign.
To see the correct currency sign, you can export your report to excel and edit it. Here's how:
In your QuickBooks Online company, go to the Hamburger icon.
Select Reports and search Customer Balance Summary Report.
Select in the Customize button and click the Filter.
Tick the box for the Customer and choose the Customer's name.