I've deleted an invoice to a customer by mistake, can I bring it back?

Hello, julian22! Welcome to the QuickBooks Community forum. I understand that you need to restore a deleted transaction in QBO. 

I'd like to let you know that when a transaction like an invoice is deleted, it's removed permanently from the system.

As a work around, you can run the Audit Log report. This will display details for invoices, bills, and other forms that have been erased. Referencing the Audit Log, you can view data such as the invoice number, date, customer name, line items, and other details. 

Let me also share this article as a guide on how to run the report: Use The Audit Log to Re-enter Deleted Transactions.

Once you have the information, follow the steps in this article on how to re-enter the invoice: Create Invoices In QuickBooks Online. 

While the original invoice cannot be recovered, you can manually re-enter it using the Audit Log report.

I also wanted to provide a couple of support articles in case you need any additional assistance with invoicing in QuickBooks Online:

• Set Up And Send Progress Invoices In QuickBooks Online
• Record Invoice Payments In QuickBooks Online

Feel free to review those for more tips and best practices when using the invoicing feature.

Let me know if you need anything else, julian22. We're always here to help you make sure your QuickBooks data stays complete for accurate bookkeeping.