Hi Psb2
Thank you for your response.
I'm sorry to hear that you're having difficulties on deleting the invoice. Can you try to open the invoice select on history > open the payment > right click on the payment transaction and select delete. Make sure that you've changed the Setup menu > Preferences > security > transaction can't be change. After deleting the payment, you can now go to invoice > open the invoice and delete sales. Kindly refer to the following screenshots below:


Please let me know if this still doesn't work, can you please provide me a screenshot so that I can take a look into it further.
Cheers,
Princess