Hello Eran

If the invoice is 'draft' there should be a Delete button.  If you have 'posted' it then it is committed into the system and shouldn't be routinely deleted: the act of posting an invoice is intended to lock it down.

If you are testing, I suggest you do so in a test database: go to Configuration and then Databases and you can create a copy of your current database.

James.

If you "Save & Complete" an Invoice in Workbooks, the document changes it status from DRAFT to POSTED. At this point it becomes committed to the database and thus you will not see the "Delete" button anymore.

To correct this, you may wish to consider creating a Credit Note to adjust any financial figures. This is particularly useful if you have Reports based on Accounting Documents in Workbooks that need to be accurate.