I have accidentally marked an invoice as paid, so theres a discrepancy between my QB account and my bank account. How do I undo this?

Hi there, info-thelawnfirm.

Welcome to the QuickBooks Community. I'll be sharing details to ensure you can undo a paid invoice so your QuickBooks and Bank account balance accurately.

The All Sales page in QBO displays invoices that have been created and settled. To undo the entry that has been marked as paid, I suggest going to the said tab and deleting the payment associated with the specific invoice. 

Here's how:

1. Go to Bookkeeping and select Transactions. Then, All Sales.
2. Select the invoice you want to reverse the payment of. Then, click the Edit invoice button.
3. Click on the payment link under the PAID stamp. Then, choose the date link.
4. On the Receive Payment window, check the payment linked to the invoice you want to remove the payment on.
5. Go to the bottom of the page and click More.
6. Select Delete and choose Yes to confirm the action.
7. Hit Save and close.

After that, the entry is now tagged as open and ready to link to the correct payment. Then, you can now review the QuickBooks balance and bank balance to make sure are accurate.

Lastly, you may refer to this article to see steps on how you can send statements to show customers summaries of their invoices, payments, credits, and balances: Create and send customer statements in QuickBooks Online.

Please know that whenever you need help, I'm always here to assist you further, info-thelawnfirm. Just leave a reply below and I'll get back to you as soon as I can. Take care and stay safe.