This should solve your problem but I don't know why I got different result ! Had you computed them manually ?
declare @tbl1 as table(
balance decimal(18,2),
month int)
insert into @tbl1 values (-1308998.02, 1)
insert into @tbl1 values ( -990000, 2)
insert into @tbl1 values ( 1178835.01, 3)
insert into @tbl1 values ( -188835.01, 4)
insert into @tbl1 values ( 0, 5)
insert into @tbl1 values ( 794.99, 6)
insert into @tbl1 values ( 160589.98, 7)
insert into @tbl1 values ( -159000, 8)
insert into @tbl1 values ( 1405.6, 9)
insert into @tbl1 values ( 13200, 10)
insert into @tbl1 values ( 1721328.02, 11)
insert into @tbl1 values ( 436984.49, 12)
select balance, month, (select sum(balance) from @tbl1 where month <= t.month ) [after sum] from @tbl1 t
Good Luck