If you cancel a document in Flexi but do not currently have the Elcom cash register connected — the one on which the transaction was registered when the document was created — you need to cancel the transaction in Elcom retroactively.
The cancellation is performed in Flexi — reopen the sales receipt and click on Services - Cancel - fiscal cash register. In this case, the sales receipt is sent to Elcom again with a negative value (this feature was added in version 2018.2.6).
The cancellation is performed in Elcom — via the menu and the cancellation function.