There are 2 methods to delete sale invoice, See below
NOTE : This operation can not be undo

Method 1:

Step 1: Go to Sales Invoice Menu & click on the down arrow beside the edit option

Step 2: Click on the delete invoice option

Step 3: Click on the Delete button to verify delete operation.

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Method 2:

Step 1: Go to Sales Invoice page & select the checkbox before invoice number & click on the “Delete” button at bottom of page to delete selected invoices.

Step 2: Click on the “Delete” button to verify delete operation.