How Can I Get My Code To Look For Partial Payments

The following code accepts a couple of parameters and creates a temp table to hold unpaid schedule rows (like invoices). Then is takes the payment amount passed and starts paying the oldest ones first, until it runs out of money or pays a partial. When it pays it inserts a row in the applieds table with the schedule_ID, Receipt_ID (payment id), applied amount, and applied date.

What I need help with is: Say there is a partial payment from a previous payment, I need to pay that one, but only the unpaid part. Then continue to pay other schedule rows...

I appreciate any help,

@PaymentAmount money,
@PledgeID Int,
@Receipt_ID Int

AS

-- Get unpaid reminder rows for the passed in pledge id and put them into the temp table
Declare @temp_oldest Table
(
PledgeSchedule_ID int,
ReminderDueDate datetime,
AmountDue money
)
INSERT INTO @temp_oldest SELECT PledgeSchedule_ID, PledgeDueDate, AmountDue
FROM tblPledgeReminderSchedule
WHERE (dbo.tblPledgeReminderSchedule.Pledge_ID=@PledgeID) AND (dbo.tblPledgeReminderSchedule.ReceivedDate IS NULL)
-- AND (dbo.tblPledgeReminderSchedule.PledgeDueDate < GETDATE())
WHILE((SELECT Count(*) FROM @temp_oldest)>0)
BEGIN
-- If the payment is greater or equal to the amount due for the current row, do this
IF(@PaymentAmount >= (SELECT Top 1 AmountDue FROM @temp_oldest))
BEGIN
-- Update the reminder row with todays date
UPDATE tblPledgeReminderSchedule SET ReceivedDate = GETDATE()
WHERE PledgeSchedule_ID = (SELECT Top 1 PledgeSchedule_ID FROM @temp_oldest)
-- Insert a row to track applied payment and reminder row associated
INSERT INTO tblPledgeReminderSchedule_Applieds (PledgeSchedule_ID,Receipt_ID,Applied_Amount,Applied_Date)
(SELECT Top 1 PledgeSchedule_ID,@Receipt_ID,AmountDue,GETDATE()
FROM @temp_oldest
WHERE PledgeSchedule_ID = (SELECT Top 1 PledgeSchedule_ID FROM @temp_oldest))
-- Subtract the amountdue from the paymentamount and reset it
SET @PaymentAmount = (@PaymentAmount - (SELECT Top 1 AmountDue FROM @temp_oldest))
END
ELSE
IF(@PaymentAmount < (SELECT Top 1 AmountDue FROM @temp_oldest))
BEGIN
 -- Insert a row to track applied PARTIAL payment and reminder row associated
INSERT INTO tblPledgeReminderSchedule_Applieds (PledgeSchedule_ID,Receipt_ID,Applied_Amount,Applied_Date)
(SELECT Top 1 PledgeSchedule_ID,@Receipt_ID,@PaymentAmount,GETDATE()
FROM @temp_oldest
WHERE PledgeSchedule_ID = (SELECT Top 1 PledgeSchedule_ID FROM @temp_oldest))
BREAK
END
ELSE

IF @PaymentAmount = 0
-- Delete all rows from temp table
DELETE FROM @temp_oldest
ELSE
-- Delete only the current row from the temo table
print @PaymentAmount
DELETE FROM @temp_oldest WHERE PledgeSchedule_ID = (SELECT Top 1 PledgeSchedule_ID FROM @temp_oldest)
END