PartyInvoice Table
PartyID  InvoiceNumber    Amount
  A       Invoice001       100
  A       Invoice002       200
  B       Invoice003       250 

PartyPayment tables
PartyID  PaymentNumber    Amount
   A      Pay001           150
   B      Pay002            50

Results 
PartyID  InvoiceNumber    InvAmount PendingAmount
  A       Invoice001       100         0
  A       Invoice002       200         150
  B       Invoice003       250         200 

-- Set up sample data
CREATE TABLE #PartyInvoice (PartyID  CHAR(1), InvoiceNumber VARCHAR(50), Amount MONEY)
INSERT #PartyInvoice
SELECT 'A', 'Invoice001', $100
UNION ALL SELECT 'A', 'Invoice002', $200
UNION ALL SELECT 'A', 'Invoice003', $400
UNION ALL SELECT 'B', 'Invoice003', $250 

CREATE TABLE #PartyPayment (PartyID CHAR(1), PaymentNumber VARCHAR(50), Amount MONEY)
INSERT #PartyPayment
SELECT 'A', 'Pay001', 150
UNION ALL SELECT 'B', 'Pay002', 50

-- Creae CTE
;
WITH Invoice (PartyID, InvoiceNumber, InvoiceAmount, PaymentAmount)
AS
(
	SELECT
		PartyID,
		InvoiceNumber,
		Amount AS InvoiceAmount,
		Payment.PaymentAmount
	FROM
		#PartyInvoice AS Invoice
	CROSS APPLY
	(
		SELECT SUM(Amount) AS PaymentAmount
		FROM #PartyPayment AS Payment
		WHERE Payment.PartyID = Invoice.PartyID 
	) AS Payment
)

-- Perform Select
SELECT 	
	A.PartyID, 
	A.InvoiceNumber, 
	A.InvoiceAmount,
	CASE
		WHEN SUM(B.InvoiceAmount) - A.PaymentAmount < 0 THEN 0
		ELSE  SUM(B.InvoiceAmount) - A.PaymentAmount
	END AS PendingAmount
FROM
	Invoice AS A
LEFT OUTER JOIN
	Invoice AS B
	ON A.PartyID = B.PartyID
	AND A.InvoiceNumber >= B.InvoiceNumber
GROUP BY
	A.PartyID, 
	A.InvoiceNumber, 
	A.InvoiceAmount,
	A.PaymentAmount
ORDER BY
	A.PartyID, 
	A.InvoiceNumber

-- Cleanup
DROP TABLE #PartyInvoice
DROP TABLE #PartyPayment