Dynamic Form Rule - Adding Record in Related Sub Table

I would suggest using Automations:

Online Hlep for Automations
https://help.quickbase.com/user-assistance/about_quick_base_automations.html

For the trigger, you would want "when data changes".

You can specify the fields that changed, and the filter condition for the trigger to fire - such as:
[Status] = "Complete] for the Work Order
AND
[# of Invoices] > 0

Then the action would be something like:

- Add a record in the Receivables table
- with [Date Invoiced] = [Date of Last Invoice]  (or something similar)