Kerstin,
The closest i could find in the WSDL of the Manage Invoice Request Web Service that handles this integration is "<ProposedInvoiceDate>" or possibly even "<ProposedDeviatingPostingdate>", both of which sit directly under the <CustomerInvoiceRequest> element.
For more information, go to the Service Explorer, find the ManageInvoiceRequestsIn Web Service, download the WSDL, and open it in SOAP-UI or something similar. This way you can see all the fields that you can write to, which is where i found these two elements.