Why can't I delete an issued invoice?
An issued invoice is a tax document with its own number. Deleting it would leave a gap in your numbering and your books. So it stays, and a credit note cancels it in full or in part. Open the invoice, open Manage invoice and press Create credit note. Credit all remaining cancels the whole invoice.
What about a draft?
A draft has no number yet and you can change it freely. Discard changes drops the edits you have not saved. A draft that you never issue simply stays a draft.
What about a wrong payment?
A payment can be voided. On the invoice, press Void… next to the payment. Deline asks you to confirm, because this cannot be undone; the payment then shows as Voided and no longer counts as paid.
Can I copy an invoice?
There is no duplicate button. To bill the same thing again, create a new invoice, or use a recurring invoice for repeated billing.
Explanation written from the Deline app's own screens and texts (build eba146497 (Deline cf723fe1f)); it has no steps to check.