Fix Code to Accept Negative Input for AddBusinessDays to date?

what's the issue for your function posted?

you already have this:

dateadd(DAY,num +@days,@FromDate) as b

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it should worked if @days is negative too

For negative @Days input, if I interpret your function correctly, you might need to change the code in 2 places:
1. Go back to the dates instead of checking forward

dateadd(DAY,-num +@days,@FromDate)

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to

dateadd(DAY,case when num > 0 then num +@days else -num+@days end,@FromDate)

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2. and the below WHERE condition in 2 places
where condition from

o.Holiday between a and b 

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to

((@days > 0 AND o.Holiday BETWEEN a and b) OR (@days < 0 and o.Holiday between b and a))

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to get the records fetched out..
If possible, kindly provide the table structure of holidays table so that I can check it out once whether this will work fine or not..

Thanks guys for the input. I tried your suggestion, Raja, and I'm getting a null output in both + and - days cases.

Here's the Holiday table, below, with the code with your suggested edits.

ALTER function [dbo].[DateAddWorkDay]
(@days int,@FromDate Date)
returns Date
as
begin
declare @result date
set @result = (
select b
from
(
    SELECT
    b,
       (DATEDIFF(dd, a, b))
      -(DATEDIFF(wk, a, b) * 2)
      -(CASE WHEN DATENAME(dw, a) = 'Sunday' THEN 1 ELSE 0 END)
      -(CASE WHEN DATENAME(dw, b) = 'Saturday' THEN 1 ELSE 0 END)
      -COUNT(o.Holiday) 
      as workday
    from
    (
    select 
    @FromDate as a,
       dateadd(DAY,case when num > 0 then num +@days else -num+@days end,@FromDate) as b
    from (select row_number() over (order by (select NULL)) as num
          from Information_Schema.columns
         ) t
    where num <= 100 
    ) dt
        left join Holiday o on 
		((@days > 0 AND o.Holiday BETWEEN a and b) OR (@days < 0 and o.Holiday between b and a)) 
		AND DATENAME(dw, o.Holiday) not in('Saturday','Sunday') 
    where DATENAME(dw, b) not in('Saturday','Sunday')
          and b not in (select Holiday from Holiday where Holiday between a and b) 

    group by a,b
) du
where workday =@days 


)
return @result 
end

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CREATE TABLE [dbo].[Holiday](
	[Holiday] [date] NULL
) ON [PRIMARY]

GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-01-01' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-01-01' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-09-03' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-11-22' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-11-23' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-12-25' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-12-26' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-12-27' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-12-28' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2018-12-31' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-01-01' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-05-27' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-07-04' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-09-02' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-11-28' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-11-29' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-12-25' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-12-26' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-12-27' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-12-30' AS Date))
GO
INSERT [dbo].[Holiday] ([Holiday]) VALUES (CAST(N'2019-12-31' AS Date))
GO

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Thanks very much,gentlemen, in particular Raja!
All the Best,
Steve

Welcome, Steve!!