If you can't delete a sales quick entry, you need to reverse it instead.
Reversing creates an opposing transaction that cancels the original. This:
You can't delete a sales quick entry if:
If you included the invoice on a submitted VAT Return, reverse it after removing any allocated payments.
If your invoice has a payment allocated, read unallocate a customer receipt from an invoice before following the steps in this guide.
Enter the same Net amount as the transaction your reversing.
The Net, VAT Rate and VAT columns only appear when you select a VAT scheme under Accounting dates and VAT.
You now need to allocate the original transaction to the reversal transaction you've created. Read allocate existing transactions for more information.