I tried to send an invoice and it sent it with a received payment, how did that happen?

Hello there, @wschell.

It's nice to have in the Community.  I'd be happy to help get this taken care of today.

When you emailed an invoice to your customer, and it's showing a paid status, it's possible that the Automatically apply credits in your settings is turned on. If the customer has an existing credit or overpayment, QuickBooks Online will automatically apply that credit to the newly created invoice. 

Here's how you can check: 

1. At the top right, click the Gear icon.
2. Select Account and Settings.
3. On the left pane, select Advanced.
4. Click on Automation. 
5. If the Automatically apply credits box is marked, remove the check mark. 
6. Click Save.
7. Click Done.

For more details about this, I recommend checking this article: Apply a credit memo, credit or refund to a customer.

To mark the invoice unpaid, you can remove the credit from an invoice.

Here's how: 

1. On the left pane, select Sales.
2. At the top left, select Customers.
3. Select the customer's name.
4. From the Transaction List tab, select the invoice.
5. At the top right, click the payment link.
6. Under Outstanding Transactions, unselect the invoice.
7. Under Credits, unselect the Credit Memo.
8. Click Save and close.

After that, you can resend the invoice to your customer. 

Also, if you need additional help, our phone support agents will be able to assist you in getting this resolved.

That should do it. Let me know if you have any other questions about the invoice status sent to your customers. I'll be happy to help you out.