Please, have a look at example:
DECLARE @tmp TABLE (PayDate DATETIME, PayAmount DECIMAL(8,2), Client_ID INT)
INSERT INTO @tmp (PayDate, PayAmount, Client_ID)
VALUES('2012-06-14', 2000.00, 1), ('2012-06-15', 2000.00, 1), ('2012-06-16', 2000.00, 1),
('2012-06-15', 4500.00, 2), ('2012-06-16', 4500.00, 2), ('2012-06-17', 4500.00, 2)
SELECT t1.RowNo AS OperationID, t1.Client_ID, t1.PayDate, t1.PayAmount, SUM(t2.PayAmount) AS Total
FROM (
SELECT ROW_NUMBER() OVER(PARTITION BY Client_ID ORDER BY PayDate) AS RowNo, Client_ID, PayDate, PayAmount
FROM @tmp
) AS t1 INNER JOIN (
SELECT ROW_NUMBER() OVER(PARTITION BY Client_ID ORDER BY PayDate) AS RowNo, Client_ID, PayDate, PayAmount
FROM @tmp
) AS t2 ON t1.Client_ID = t2.Client_ID AND t1.RowNo >= t2.RowNo
GROUP BY t1.Client_ID, t1.RowNo, t1.PayDate, t1.PayAmount
Result:
OperationID Client_ID PayDate PayAmount Total
1 1 2012-06-14 00:00:00.000 2000.00 2000.00
2 1 2012-06-15 00:00:00.000 2000.00 4000.00
3 1 2012-06-16 00:00:00.000 2000.00 6000.00
1 2 2012-06-15 00:00:00.000 4500.00 4500.00
2 2 2012-06-16 00:00:00.000 4500.00 9000.00
3 2 2012-06-17 00:00:00.000 4500.00 13500.00
Is that what you want?