That's a brilliant suggestion, @chris194.

Yes, you're correct. QuickBooks Online (QBO) will auto-generate an invoice number when you create a new transaction even if the Custom transaction numbers feature is turned on. 

I've come up with a workaround to get you back up and running. To prevent duplicates or invoice number mix-ups, you can run or print the Invoice List report. Then, manually edit the invoice number field and compare it with the said report.

There are two ways to achieve this depending on your preference. First, you can just run the said report by logging in to your account separately using an incognito or a different browser. 

On the first browser, you can create your invoices and edit the transaction numbers manually. Then, pull up the report on the other browser so you can compare it to the newly-created invoice entries.

Here's how:

1. Go to the Reports menu.
2. Select the Invoice List report from the Who owes you section.
3. Set the Report date to All Dates, and choose Transaction Type under the Group by drop-down.
4. Click the small Gear icon, and make sure that the Num box is checked.
5.  Tap Run report.
   

The second option is to print the said report to keep a hard copy. Then, compare it to the invoice numbers for the transactions you'll be creating. To do this, you can follow the steps above to run your report.

To print your report:

1. Click the Print icon at the top.
2. Select the Print button, and then make the needed modifications.
3. Click Print again to continue.
   

You can also customize your invoices to personalize them. Once you receive the payments from your customers, you can enter them in QBO to keep your record up-to-date and accurate.

I'm always around to help if you some follow-up questions about this. Have a good one.