I don't know  whether cast is better or convert is better. 

CAST (SUM(Principal1) AS DECIMAL(5,2) ) + CAST ( SUM(BankInterest) AS DECIMAL(5,2)) + CAST(SUM(TAltrate) as DECIMAL(5,2) AS Amount
or 
CAST (SUM(Principal1) + SUM(BankInterest) + SUM(TAltrate) + SUM(PanaltyCharge) AS DECIMAL(5,2))
Above query gives almost similar results/value...