Total Assets always show null. Can any one help me get the right value

SELECT 'ASSETS :' AS [Account Name],'Totals:' AS [$]

UNION ALL

SELECT COA.Account_Name AS 'ASSET',CAST(SUM(Debit-Credit)AS varchar)AS [$]
FROM COA JOIN CashBook ON COA.Account_Name = CashBook.Account_Name
WHERE COA.Account_Name IN ('Cash-in- Hand')
GROUP BY COA.Account_Name

UNION ALL

SELECT COA.Account_Name AS 'ASSET',CAST(SUM(Debit-Credit)AS varchar)AS [$]
FROM COA JOIN AccountReceivable ON COA.Account_Name = AccountReceivable.Account_Name
WHERE COA.Account_Name IN ('Account Receivable')
GROUP BY COA.Account_Name

UNION ALL

SELECT COA.Account_Name AS 'ASSET',CAST(SUM(Debit-Credit)AS varchar)AS [$]
FROM COA JOIN Allowance ON COA.Account_Name = Allowance.Account_Name
WHERE COA.Account_Name IN ('Allowance for Doubtful Account')
GROUP BY COA.Account_Name
UNION ALL

SELECT '','------------'

UNION ALL

SELECT 'Total Assets',CAST(SUM(CashBook.Debit- CashBook.Credit+ AccountReceivable. Debit- AccountReceivable.Credit+ Allowance.Debit-Allowance.Credit) AS varchar) AS [$]
FROM COA JOIN CashBook ON COA.Account_Name = CashBook.Account_Name JOIN AccountReceivable ON COA.Account_Name = AccountReceivable.Account_Name JOIN Allowance ON COA.Account_Name = Allowance.Account_Name
WHERE COA.Account_Name_Cate IN ('Assets')