How to display the remaining debt of the customer on an ODT invoice?

Hello, I apologize for my limited English, so this message might be a bit hard to understand.
I’m using an ODT Template to create invoices. I want to print the amount that the customer currently owes on each invoice.

I have created an extra field within the “societe” module, with its code being “cusdebt". That value is calculated by fomular: $object->getOutstandingBills($objectoffield->id, 0)[‘opened’].
On page /societe/card.php?socid=xxx, that value is showed up. But on the ODT file, when I use tag {company_options_cusdebt}, it show blank.

Please guide me on how to do it. Thank you, everyone.