def initiate(request_body, response_body, storage):
    """
    This callback function is called on a request to the `/v1/payments/initiate` endpoint.
    It stores a request body into internal storage to modify accounts after confirmation.
    :param request_body: a request body
    :param response_body: a mocked response body
    :param storage: the internal storage
    :return: a response body without modifications
    """
    storage[response_body['paymentId']] = request_body
    return response_body