CREATE FUNCTION fn(@AddCondition varchar( 8000 ))
RETURNS INT
AS
BEGIN
    DECLARE @SQL varchar( 8000 ), @CNT int
    SET @SQL = 'SELECT COUNT(*) FROM Tbl WHERE ' + @AddCondition
....
   RETURN @CNT
END

CREATE PROCEDURE func
    @Condition varchar( 8000 ),
    @Date datetime
RETURNS INT
BEGIN
DECLARE 
	@date_str varchar( 8000 ),
	@date_bom_str varchar( 8000 ),
	@result int

SET @date_str = CONVERT(varchar( 30 ), @date,  112 )

SET @date_bom_str = CONVERT(varchar( 30 ), DATEADD(day, -DATEPART(day, @date), @date),  112 )

DECLARE @SQL nvarchar( 4000 )
SET @SQL = 
N'
SELECT  @result = count(*)
FROM DD
INNER JOIN MD on DD.InstrumentID = MD.InstrumentID and MD.Date = '''+@date_str+'''
INNER JOIN BOM on DD.InstrumentID = BOM.InstrumentID and BOM.Date = '''+@date_bom_str+'''
WHERE
'

SET @SQL = @SQL + @Condition

EXEC sp_executesql @SQL, N'@result int output', @result = @result output

RETURN @result
END

SELECT ConditionID, FOUND = func(Condition, '2001-01-01')
FROM Conditions