SQL Merge with Partial Matching tables - sql-server

Question

I have investigated lots of posts relating to SQL merging of two tables, and am very familiar with merging in SQL, but this one has me stumped. Here is the scenario: I have a table of values which are stored by Grade and Currency, i.e.
GradeID = 3, Rate = 175, CurrencyID = 5.
There are five grades, a different value for each and these are the default values for each grade in GBP (CurrencyID = 5).
I have another table with an extra column of Fiscal Period, which is a 1st of the month date to which the rate applies, i.e.
GradeID = 3, Rate = 150, FiscalPeriodID = 95, CurrencyID = 5.
Not all values for each grade, fiscal period and currency will be different from the default, so users will only enter differences where necessary. I have a Windows form for the user to view and update these values, but am struggling to show the results of both tables together.
The ideal solution for a case where there is no stored data, i.e. only the default values apply, is that the screen will show the default values at the current fiscal period i.e. 95 = 01/11/2017, but where there are values which have been entered, the table will show the changed rows by grade but grouped by each fiscal period which has a non-default value.
The first part of setting the current fiscal period for the default values is relatively easy:
SELECT [RateID], [GradeID], [ChargeRate],
(SELECT [FiscalPeriodID] FROM [dbo].[FiscalPeriod] WHERE MONTH([FiscalPeriod]) =
MONTH(GETDATE()) AND YEAR([FiscalPeriod]) = YEAR(GETDATE())) AS [FiscalPeriodID]
,[CurrencyID]
FROM [dbo].[DefaultChargeRateByLevel]
But what I'd like to do is perform one query that does either this simple selection (when no results returned for any fiscal period) or a merge of the default and updated values to show groups of 5 values per fiscal period.
My initial thought was to populate two temporary tables with the results from each query then perform a merge, but I can't figure out how to populate the fiscal period for each 'group of values. This is what i have so far, but it fails with the following error: Cannot insert the value NULL into column 'FiscalPeriodID', table 'tempdb.dbo.#TempTarget
IF OBJECT_ID('tempdb..#TempTarget') IS NOT NULL
DROP TABLE #TempTarget
IF OBJECT_ID('tempdb..#TempSource') IS NOT NULL
DROP TABLE #TempSource
SELECT
[GradeID]
,[ChargeRate]
,[FiscalPeriodID]
,[CurrencyID]
INTO #TempTarget
FROM (SELECT
[GradeID]
,[ChargeRate]
,[FiscalPeriodID]
,[CurrencyID]
FROM
[dbo].[ChargeRateByProjectAndLevel]) AS A
SELECT * FROM #TempTarget
SELECT
[GradeID]
,[ChargeRate]
,[FiscalPeriodID]
,[CurrencyID]
INTO #TempSource
FROM (SELECT
D.[GradeID]
,D.[ChargeRate]
,M.[FiscalPeriodID]
,D.[CurrencyID]
FROM
[dbo].[DefaultChargeRateByLevel] AS D
LEFT JOIN #TempTarget AS M ON M.[GradeID] = D.[GradeID]
AND M.[CurrencyID] = D.[CurrencyID]
) AS B
SELECT * FROM #TempSource
/* Now merge rates data from temp table to FxRates table */
MERGE #TempTarget AS [Target]
USING #TempSource AS [Source]
ON [Source].[GradeID] = [Target].[GradeID]
AND [Source].[CurrencyID] = [Target].[CurrencyID]
WHEN NOT MATCHED BY TARGET THEN
INSERT (
[GradeID]
,[ChargeRate]
,[FiscalPeriodID]
,[CurrencyID]
)
VALUES (
[Source].[GradeID]
,[Source].[ChargeRate]
,[Source].[FiscalPeriodID]
,[Source].[CurrencyID]
);
SELECT * FROM #TempTarget