Why does the invoice amendment show the message ‘VTRCPCON – Receipt Already Confirmed!’ ?

Suggestions :
The system allows to take an advance receipt and then users make the invoice amendment. Once the invoice is adjusted against advance receipt then the system will not allow doing any invoice amendments and show the message ‘VTRCPCON – Receipt Already Confirmed!’ or ‘Receipt for this Invoice has already been confirmed.’ So users need to ensure if all the receipts which are received in advance are adjusted for the invoice or not before amending.