#1 Делает сразу массив, передавай куда нужно и работай!

SELECT
  `invoiceNumber`, `employee`
FROM (
  SELECT
    `date`, `employee`, (@cnt := @cnt + 1) AS `invoiceNumber`
  FROM (
    SELECT
      `date`, `employee`
    FROM
      `expense`
    WHERE
      `date`
      BETWEEN
        '2017-11-01'
      AND
        '2017-11-08'
    GROUP BY `date`, `employee`
  )
  AS `list`
  CROSS JOIN (
    SELECT
      @cnt := 0
  )
  AS `invoiceNumber`
)
AS `list`
WHERE `date` = '2017-11-03'

//

#2 Просто все записи с начала месяца без логики

SELECT
  `date`, `employee`
FROM
  `expense`
WHERE
  `date`
  BETWEEN
    '2017-11-01'
  AND
    '2017-11-08'
GROUP BY `date`, `employee`