Hello there, @karen_labase.
I'd like to share some information on why the option to choose a foreign balance is unavailable in the Transaction Report.
I can see how it would be useful to have the foreign balance in the transaction report. However, all reports in QuickBooks Online is on its first version as of this time and we're developing it to edit/customize all reports to have easy access in QBO.
For your future reference, you might also want to read this article to learn more about the customization of reports: How to Customize Reports.
Please know that you can always get back to us if you have any questions about QBO.