Here's an example:

I have $ 1000 credit (negative figure since its a credit) in a cell
called "unallocated
Cash" (cash received but needs to be allocated to the different "aged
debts" mentioned below).

Next I have the following amounts (debit balances, so positive
figures)in different cells in a row as follows:
Current, not due for payment = $ 25
Due for payment = $ 250
30 days overdue = $ 50
60 days overdue = $ 75
90 days Overdue = $ 475
120 days Overdue = $ 150
150 days overdue = $ 25
180 days overdue = $ 35

I want to allocate the $ 1000 sitting in the "unallocated Cash"
to pay the various overdue amounts starting with the oldest debt first
(in this example its $ 35 in the "180 days overdue" cell.  If there is
still money (a credit balance), the next Overdue should be tackled
(i.e. the 150 days overdue cell).  This should repeat itself till all
the money in the "unallocated cash" cell has been used to cancel the
debts in the various aged amounts.  If all of the aged balances are
"recovered" and there is still a credit balance, this should be in the
"due" cell (or column), but in the final result, the unallocated cash
colum (or cell) should be NIL>

Ok enter the data as follows:

____ 25
____ 250
____ 50
____ 75
____ 475
____ 150
____ 25
____ 35
1000

where the underscores are empty cells. 

Now, in the cell to the left of the 35 at the bottom, enter this equation:
=B9-A8

This is assuming the first 25 is at cell A1.

Then select the cell with formula, it should be B8, and drag the
little "plus" sign upward to cell B1.

That should do it.

You will get something like this:

25	-85
250	-60
50	190
75	240
475	315
150	790
25	940
35	965
	1000

I don't think that really does what bob2005 wants.

Bob2005, you've specified that the values for each period are in a
row. I'm going to assume that this is one of many rows and that we
want to create a new row with the updated values.

For this example I'm also going to assume that the cash received is in
column A, current in B, due in C, 30 days in D, etc. I'll also assume
that the present data is in row 2, including the -1000 in cell A2.

Cell A3 is obviously just going to be 0, that's the whole point, so no
formula is needed for that (when cash is received in the next period
you can just overwrite this value and repeat the whole process).

Cell C3 should contain this formula:
=IF(D3=0,MAX(SUM(C2:$I2)+$A2,0),C2)
Fill this through to cell I3 (assuming column J is blank). If column J
is not blank, change the formula in I3 to
=MAX(I2+$A2,0)

Cell B3 is a bit different because we may have a negative balance
showing if there was a large enough credit to cover all outstanding
debts. But the formula is simple:
=SUM(A2:I2)-SUM(C3:I3)

Some of these formulas will need to be changed if your spreadsheet is
not laid out in the manner I have assumed. If you need any help with
the changes, please let us know.