Thanks for visiting the QuickBooks Community, @marthaw,
I can help you resolve this billing question in QuickBooks.
Creating a journal entry crediting Accounts Payable will increase the liability balance. Even if you make a reversal, this entry will show as outstanding balance because we can't apply a payment towards a JE.
To resolve this, you will need to delete the journal for September and recreate it as a bill. A bill works similarly as creating a JE, hence, its transaction journal credits A/P and debits the bank.
To delete a journal entry, click the Delete button at the top.
Once done, create a bill dated for September using the Vendors tab and selecting Enter Bills. See this:
When you're done, link the journal entry payment for October by clicking the Pay Bill option at the top of the bill.
To use the journal entry as payment, click on Set Credits. Mark the JE for October under Credits tab. Once you're done, press Pay Selected Bills. See this:
After doing these steps, your A/P balance should be all set. For your guide with handling bills, see this resource guide: Pay bills in QuickBooks Desktop - Intuit
Let me know if you have follow-up questions about this topic. I'll be more than happy to share some more information about the tasks in QuickBooks. Have a nice day!