isNull with Cast in Sqlserver

Select        cast(isNull(FR1.Collection,0) as decimal(20,2)) as Collection,
              cast(isNull(FR2.ServiceTax,0) as decimal(20,2)) as ServiceTax,
              cast(isNull(BD.BadDebt,0) as decimal(20,2)) as BadDebt
     From  

(select SUM(CurrentPaid) AS Collection from FeeReceipt where FranchiseCode='DL') FR1,

(select SUM(STax) AS ServiceTax from FeeReceipt where FranchiseCode='DL') FR2,
(select SUM(AMOUNT) as  BadDebt from BadDebt where FranchiseCode='DL'
and VDATE between CONVERT(VARCHAR(15),'03/01/2013',103) AND CONVERT(VARCHAR(15),'10/01/2013',103)) BD