I can share some ideas on how to unlink the NSF payment to the original invoice. @audreyd08.

Transactions in deposits made from Undeposited Funds but not always are considered closed. The error would indicate that you are attempting to change a total of payment already on a deposit. To get past this you must open the transaction, and in some manner remove the payment for editing, change it and then add it back. 

To delete the deposit, here's what we need to do: 

1. Click Accounting in the left navigation bar.
2. Select Chart of Accounts.
3. Look for the account and click View Register.
4. Find the Deposit and right-click Delete.
   
   
5. If you want to delete a specific payment, select it from the list.
6. Now click the Edit tab at the top menu bar.
7. Select Delete Line.
8. Select Yes to confirm your changes.

Please note the delete line option will only show if there are multiple payments on a deposit transaction. Then, you can now unlink or delete the payment from the invoice tagged as Open. 

1. Go to the Sales menu.
2. Select the All Sales section. 
3. Find the payment transaction. Then, click it.
4. You'll be routed to the Received Payment page.
5. Click More at the bottom and choose Delete.
6. You'll be prompted with Are you sure you want to delete this? select Yes.

In addition to that, I'll be adding this link to help you record invoice payments in QuickBooks Online. 

Keep in touch with us if you need a hand with managing sales transactions or any QuickBooks-related. Just leave a comment below, and I'll be sure to answer them. Have a good one.