You can't unallocate quick entries from the Quick Entries area. Instead, unallocate the transaction from the customer receipt within Banking.

Once you unallocate the transaction, the sales quick entry invoice becomes outstanding. The customer receipt remains on the customer's account as a payment on account.

1. From Banking, open the bank account with the allocated receipt.
2. Find the customer receipt in the bank Activity.
3. Select the Customer Receipt to open its details.
4. Untick the checkbox on the invoice you want to unallocate.
   
   TIP:

   The quick entry invoice transaction type is QE-Inv.

5. Select Save.
6. Then from the Confirm window select Yes to save the unallocated amount as a payment on account.

'You can't edit this transaction as it has an associated payment or refund'

You'll see the above message when you save a customer receipt on your customer's account, and then you allocate it using account allocation. 

To unallocate these types of allocations, read break a customer allocation.

Unallocate a customer receipt from a sales invoice