Make use of additional HTTP response code 428 Precondition Required defined in RFC6585

Insert the following paragraph after the first paragraph of section 8.2.4: If an entity type or at least one of its properties is marked with ConcurrencyMode="Fixed" and the client does not specify an If-Match request header in a Data Modification Request or Action Request, the server MUST respond with a '4xx' response code and SHOULD use '428 Precondition Required'. It MUST ensure that no data is modified as a result of the request.