Follow-up question, Dynamic PIVOT query

Previous question here.

In the previous question, I forgot to mention that my production table was not exactly as listed because I assumed I could make the modifications I needed to the final query to pass in an additional variable @Product (values of 'Gas', 'Oil', 'Water') and select the appropriate column using a Case statement.  So, the following query works.

SET DATEFORMAT mdy;

Declare @Product nvarchar(5)
Declare @StartDate datetime2
Declare @EndDate datetime2
SET @Product = 'Gas'
Set @StartDate = '2014-09-01'
SET @EndDate = '2014-09-30'

--Define the string of dates to use in the Pivot clause
DECLARE @docDatesTable TABLE
(
	[docDate] Datetime
);

INSERT INTO @docDatesTable
SELECT DISTINCT docDate FROM tbl_sysProduction
WHERE docDate >= @StartDate AND docDate < DATEADD(day, 1, @EndDate);

--Build the String containing the date values to be used in the pivot
DECLARE @docDates VARCHAR(MAX);
SELECT @docDates = COALESCE(@docDates + '],[', '') +
		  (LEFT(CONVERT(VARCHAR, [docDate], 101),6) + RIGHT(YEAR([docDate]),2))
FROM @docDatesTable ORDER BY [docDate]

--Build the dynamic SQL String
DECLARE @sql NVARCHAR(MAX);
SET @sql = 'SELECT * FROM (
SELECT IsNull(W.Bolo_ID, F.Bolo_ID) as Bolo_ID
, IsNull(W.Common_Name, F.Facility_Name) as Well_Facility
, P.[Entity_ID]
, P.[docDate]
, Volume = [Gas] 
FROM tbl_sysProduction as P
LEFT JOIN tbl_sysWells as W ON P.Entity_ID = W.Entity_ID
LEFT JOIN tbl_sysFacilities as F on P.Entity_ID = F.Entity_ID
) AS [SubTable]
PIVOT
(
     SUM([Volume])
FOR [docDate] IN ([' + @docDates + '])
) AS [Pivot]
ORDER BY Bolo_ID, Well_Facility;
';

EXEC sp_executesql @sql;

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But when I replace this line:

, Volume = [Gas]

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with:

, Volume = CASE WHEN ' + @Product + ' = ''Gas'' THEN [Gas]
                WHEN ' + @Product + '= ''Oil'' THEN [Oil]
                WHEN ' + @Product + '= ''Water'' THEN [Water]
                ELSE NULL END

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I get an error message:
Msg 8114, Level 16, State 5, Line1
Error converting data type varchar to float

I also tried it with:

, Volume = CASE WHEN @Product = ''Gas'' THEN [Gas]
                WHEN @Product = ''Oil'' THEN [Oil]
                WHEN @Product = ''Water'' THEN [Water]
                ELSE NULL END

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but that raised an error also:
Must declare variable "@Product"

So, the question is, how do I pass in a value for the Product (@Product) and retrieve the correct column from tbl_sysProduction based on that value in the dynamic SQL Statement?