declare @table table ([Name] varchar(256), OnDate datetime, Summa int, Pogash int,OffDate datetime)
INSERT INTO @table([Name], OnDate, Summa, Pogash,OffDate)
SELECT 'Соколов Алексей','20080501',15000,5000,'20080708'
UNION ALL
SELECT 'Соколов Алексей','20080501',15000,8000,'20080910'
UNION ALL
SELECT 'Соколов Алексей','20080501',15000,2000,'20081015'
SELECT [Name], OnDate, Summa,
(SELECT sum(Pogash) FROM @table as t1 WHERE t1.OffDate<=t0.OffDate AND t1.[name]=t0.[name])
,OffDate FROM @table as t0