If you only want Slip Numbers then you can follow the below approcach

(
From item in dt_Excel1.AsEnumerable.Select(Function(r) r("slipnumber")).ToArray.Except(dt_Excel2.AsEnumerable.Select(Function(r) r(0)).ToArray)
select dt_Output.Rows.Add(item)
).CopyToDataTable
Hope it helps.
