is this close?

--code based on ChrisM fine work at http://www.sqlservercentral.com/Forums/Topic1731617-391-1.aspx

CREATE TABLE #t1

(TransactionID INT,

Customer INT,

TransactionType VARCHAR(10),

[Date] DATE,

Amount MONEY,

CreditID INT

);

INSERT INTO #t1 VALUES(1, 1, 'Credit', '1/1/16', -20, 1);

INSERT INTO #t1 VALUES(2, 1, 'Debit', '1/2/16', 20, NULL);

INSERT INTO #t1 VALUES(3, 1, 'Credit', '1/3/16', -100, 2);

INSERT INTO #t1 VALUES(4, 1, 'Debit', '1/4/16', 50, NULL);

INSERT INTO #t1 VALUES(5, 1, 'Debit', '1/5/16', 50, NULL);

INSERT INTO #t1 VALUES(6, 1, 'Credit', '1/6/16', -50, 3);

INSERT INTO #t1 VALUES(7, 1, 'Debit', '1/7/16', 20, NULL);

INSERT INTO #t1 VALUES(8, 1, 'Credit', '1/8/16', -50, 4);

INSERT INTO #t1 VALUES(9, 1, 'Debit', '1/9/16', 20, NULL);

WITH

Debits AS (

SELECT debitdate = date,

DebitID = TransactionID,

DebitAmount = Amount,

[from] = ISNULL(LAG([to], 1) OVER(ORDER BY TransactionID), 0),

[to]

FROM

(

SELECT *,

[to] = SUM(Amount) OVER(ORDER BY TransactionID)

FROM #T1

WHERE TransactionType = 'Debit'

) d

)

,

Credits as (

SELECT CreditId,

PaymentID = TransactionID,

CreditAmount = -Amount,

[from] = ISNULL(LAG([to], 1) OVER(ORDER BY TransactionID), 0),

[to]

FROM

(

SELECT *,

[to] = SUM(-Amount) OVER(ORDER BY TransactionID)

FROM #T1

WHERE TransactionType = 'Credit'

) c

)

, Results as (

SELECT c.CreditId,

debitdate,

Balance = CASE

WHEN c.[to] > d.[to]

THEN c.[to] - d.[to]

WHEN d.[to] IS NULL

THEN c.CreditAmount

ELSE 0

END,

ROW_NUMBER() OVER(PARTITION BY c.CreditId ORDER BY DebitId DESC) rn

FROM Debits d

FULL OUTER JOIN Credits c ON c.[from] < d.[to] AND c.[to] > d.[from]

)

SELECT CreditId,

Balance AS remaining_balnce,

debitdate AS last_redeemed_date

FROM results

WHERE rn = 1

ORDER BY creditid;

DROP TABLE #t1