Can I cancel a payment?I created an invoice and hit save to finish later and it is now marked paid.

Hello, Michelle. I can guide you on how to delete the payment in QuickBooks Online (QBO).
 

If the Automatically apply credits is enabled and your customer has an existing credit, your invoice will be mark as paid after saving it. Let's go to the Account and Settings and disable it so we can delete the payment. 

Here's how: 
 

1. Select Account and Settings from the Gear icon.
2. Go to the Advanced menu.
3. From the Automation section, click the pencil icon.
4. Switch off the Automatically apply credits button.
5. Select the Save button.
6. Click Done.
    

After turning it off,  let's go to Sales and delete the payment attached to the invoice that was marked as paid. 
 

In addition, check out this article for more information managing your invoices using the new the interface: Customize invoices, estimates, and sales receipts in QuickBooks Online. 

Please let us know if you have further questions about payments and invoices inside QuickBooks. We're always here to help. Have a good one.