DECLARE @sample TABLE
(
	TransactionID 	int,
	TransactionDate datetime,
	Amount 		decimal(10,2),
	FromAccountID 	int,
	ToAccountID	int
)
INSERT INTO @sample
SELECT	1, DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()), 0), 100.00, 1000, 1001	UNION ALL
SELECT	2, DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()), 0),  40.00, 1002, 1000	

SELECT	AccountID, Amount = SUM(Amount)
FROM
(
	SELECT	AccountID	= FromAccountID,
		Amount		= -Amount
	FROM	@sample s
	WHERE	s.TransactionDate	= DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()), 0)
	
	UNION ALL
	
	SELECT	AccountID	= ToAccountID,
		Amount		= Amount
	FROM	@sample s
	WHERE	s.TransactionDate	= DATEADD(DAY, DATEDIFF(DAY, 0, GETDATE()), 0)
) a
GROUP BY AccountID

/*
AccountID   Amount                                   
----------- ------
1000        -60.00
1001        100.00
1002        -40.00

(3 row(s) affected)
*/