For API accounting:documents (1.0) you transfer a document by using:
POST /accounting/v1/clients/{client-id}/documents
The client is specified by <consultant_number>-<client_number> from the list of clients accessible to the user. To get the consultant number and the client number, you must use:
GET /accounting/v1/clients/
If the customer records the consultant number and the client number by himself, you have to check the permission. Please use therefore:
GET /accounting/v1/clients/<consultant_number>-<client_number>
When you request the access token for offline_access you have to send at least the scope accounting:documents offline_access datev:iam:client:<consultant_number>-<client_number>.
If you store the refresh token in your application, you can transfer a document for the given client id in for at least 2 years (the refresh token's lifetime).