An existing Invoice with the specified InvoiceID could not be found.

There are two situations when the error "An existing Invoice with the specified InvoiceID could not be found" can occur:
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1. When a document has been initially exported as a Purchase Invoice and then switched to Sales and re-exported again as a Sales Invoice.

2. When exporting a document to a new Xero organisation, but the document was already exported to another Xero previously (i.e document was moved between folders connected to different Xero organisations).

Note: When a document with the status ‘ready’ is moved across folders, there should not be such problem.
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