Thank you, Magnus! Yesterday I was relying on the GUI to find the required properties of an invoice. Sadly, the operation is not presented in a way to makes this clear. (It would be lovely if the Swagger mentioned which parameters are required instead.) I am able to create an invoice without specifying inventoryNumber or description (see my new request body below). The sub account definition in my request body was invalid, but I corrected it based on the response I got when fetching an existing invoice (which was created through the GUI). Request body { "cashDiscountDate": { "value": "2020-10-27T00:00:00.000Z" }, "creditTermsId": { "value": "30" }, "customerNumber": { "value": "20000" }, "documentDate": { "value": "2020-10-27T00:00:00.000Z" }, "documentDueDate": { "value": "2020-11-26T00:00:00.000Z" }, "financialPeriod": { "value": "202010" }, "invoiceLines": [ { "accountNumber": { "value": "8100" }, "operation": "Insert", "quantity": { "value": 100 }, "subaccount": [ { "segmentId": 1, "segmentValue": "0" } ], "unitPriceInCurrency": { "value": 5 }, "vatCodeId": { "value": "1" } } ] }
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