SQl Running Totals And Subtracting

I have a table for which I need a €œspecial€? running total. More specifically there should be a close communication between the credit and the debit column. The scenario should be in the following order





Credit

Debit

Balance credit

Balance debit


6

0

6

0


5

0

11

0


0

4

7

0


0

9

0

2


3

0

1

0


0

5

0

4


0

2

0

6
I have to point out that this is done in a grouped way where the CustormerID is the grouped clause. Now I have achieved this to a point where the calculations work only if the credit column is bigger than 0, but when the account starts with debit I get only sums of the column not the needed subtractions. What am I missing? Thanks in advance.
DECLARE @PrevRunBal MONEY --Overall running total
SET @PrevRunBal = 0
DECLARE @PrevGrpBal MONEY --Running total resets when account changes
SET @PrevGrpBal = 0
DECLARE @PrevRunCnt INT --Overall running count (ordinal rank)
SET @PrevRunCnt = 0
DECLARE @PrevGrpCnt INT --Running count resets when account changes
SET @PrevGrpCnt = 0
DECLARE @PrevAcctID INT --The "anchor" and "account change detector"
SET @PrevAcctID = 0
changes
SET @PrevGrpBalP = 0
update Temp

SET --===== Running Total
@PrevRunBal = RunBal = @PrevRunBal + Credit,

--===== Grouped Running Total (Reset when account changes)
@PrevGrpBal = Bcredit = CASE

WHEN CustomerID = @PrevAcctID
THEN CASE
WHEN CREDIt > DEBIT or CREDIT = 0
THEN (@PrevGrpBal+CREDIT)-(DEBIT)
WHEN DEBIT>CREDIT or DEBIT =0
THEN (@PrevGrpBal+DEBIT)-DEBIt
END
ELSE CASE
WHEN CREDIT >DEBIT
THEN (CREDIT)
WHEN DEBIT > CREDIT
THEN DEBIT -- restarts from 0 if only 1 rec.
END

END,


--===== Running Count (Ordinal Rank)
@PrevRunCnt = RunCnt = @PrevRunCnt + 1,
--===== Grouped Running Total (Ordinal Rank, Reset when account changes)
@PrevGrpCnt = GrpCnt = CASE
WHEN CustomerID = @PrevAcctID
THEN @PrevGrpCnt + 1
ELSE 1 -- Restarts count at "1"
END,

--===== "Anchor" and provides for "account change detection"
@PrevAcctID = CustomerID