need spill formula

[Scott R]

Rows 2:8 show inputs. Rows 10:14 show desired output--I'd like a dynamic array formula in B10 that spills to B10:C14 (two columns in this example but I really need to spill to 40+ columns).

B2: Item1 needs 10,000 of purchases.
B8: Total purchases are 50,000.
B10: 10,000 of the purchases are allocated to Item1, so the new balance for Item1 is 0, and there are 40,000 of purchases to allocate to Items 2, 3, and part of 4.
Repeat for each Month (column).

So it's basically Rows 2:6 minus Row 8 = Rows 10:14, where no values in 10:14 would be negative and allocation order would cascade from Item1 to Item5.

Book2
	A	B	C
1		Month1	Month2
2	Item1 begin bal	10,000	10,000
3	Item2 begin bal	11,000	23,000
4	Item3 begin bal	26,000	21,000
5	Item4 begin bal	7,000	11,000
6	Item5 begin bal	29,000	15,000
7
8	tot purchased	50,000	20,000
9
10	Item1 end bal	0	0
11	Item2 end bal	0	13,000
12	Item3 end bal	0	21,000
13	Item4 end bal	4,000	11,000
14	Item5 end bal	29,000	15,000
Sheet1

 

[hagia_sofia]

Here's what I landed on:
BYCOL spills right, VSTACK spills down (thx for the tip, Sofia!)

=LET( ary, B2:C6,     purch, B8:C8,
           DROP( REDUCE( "", SEQUENCE( ROWS( ary )), LAMBDA(a,b, VSTACK( a,
                           LET( prevCumul, IF( b = 1, SEQUENCE( , 2, 0, 0 ), BYCOL( TAKE( ary, b - 1 ), LAMBDA(c, SUM( c )))),
                                    bbal, CHOOSEROWS( ary, b ),
                                    ebal, IF( prevCumul > purch, bbal, bbal - purch + prevCumul ),
                                    IF( ebal < 0, 0, ebal ))))),
   1 ))

Out of curiosity I tested the formula and it seems that there is some issue with the first row when the first value is equal or greater than total it adds the column number to the sum.