Format Value To Currency

CREATE FUNCTION format_currency (@monetary_value decimal(20,2) ) returns varchar(20)
as
begin
declare @return_value varchar(20)
declare @is_negative bit
select @is_negative = case when @monetary_value<0 then 1 else 0 end

if @is_negative = 1
set @monetary_value = -1*@monetary_value

set @return_value = convert(varchar, isnull(@monetary_value, 0))


declare @before varchar(20), @after varchar(20)

if charindex ('.', @return_value )>0 
begin
set @after= substring(@return_value,  charindex ('.', @return_value ), len(@return_value))
set @before= substring(@return_value,1,  charindex ('.', @return_value )-1)
end
else
begin
set @before = @return_value
set @after=''
end
-- after every third character:
declare @i int
if len(@before)>3 
begin
set @i = 3
while @i>1 and @i < len(@before)
begin
set @before = substring(@before,1,len(@before)-@i) + ',' + right(@before,@i)
set @i = @i + 4
end
end
set @return_value = @before + @after

if @is_negative = 1
set @return_value = '-' + @return_value

return @return_value 
end