I've got some information for you about unapplying payments in QuickBooks, @Plinkr.

If you’re referring to jamespaul recommendation above, yes, all invoice payments you’ve recorded in QuickBooks Desktop will be deleted.

Here’s how:

1. Select the Customers menu, then click Customer Center.
2. Locate the customer's name.
3. In the Transaction tab, select the payment.
4. Uncheck the entry, then press Save &Close.
5. Click Yes in the prompt message to save the changes.
   

You can turn off the Automatic apply payments feature in the Preference so any credits won’t be directly allocated to the next invoice issued to the customers' account. I'm glad to show you the steps.

1. Go to the Edit menu.
2. Click Preferences.
3. Select Payments, then Company Preferences.
4. Uncheck the box in the Automatically apply payments.
5. Press OK after.
   

You might want to read this article that contains steps to remove the customer or vendor credit: Remove or unapply a credit from an invoice or bill.

I've also included this helpful reference to help manage your customer transactions: Learn the different ways you can track customer transactions in QuickBooks Desktop.

If you have any other questions or concerns besides unapplying payments, just let me know. I'll do my best to help you out. Take care!