Something like this:
;WITH CTE AS
(
SELECT [EmployeeID],
[Posting_Date]
,[Quantity]
,ROW_NUMBER() OVER (PARTITION BY [EmployeeID], [Posting_Date] ORDER BY [Quantity]) RN
FROM [Fact table]
)
SELECT t1.[Posting_Date]
,t1.[EmployeeID]
,t1.[Quantity]
,CASE WHEN t1.RN = 1 THEN t2.[Monthbase] ELSE 0 END [Monthbase]
FROM CTE t1
LEFT JOIN [Time table] t2
ON t1.[Posting_Date] = t2.[Posting_Date]