The response of the callback will contains all the fields that are on the transactions endpoint in sikka api ?

No, the response will contain 

{

    "event": "transaction",

    "office_id": "D18620",

    "practice_id": "1",

    "generated_time": "2019-03-14 15:42:42"

}

See here: https://apidocs.sikkasoft.com/#webhooks