This might help with adding some tweaks to the report. 
https://forum.sambapos.com/t/custom-reports-customer-balance/7247/33
https://kb.sambapos.com/en/9-8-1-remaining-balanced-customers-report/
https://kb.sambapos.com/en/2-4-12-d-how-to-create-custom-report-and-statements/