Using Arrayformula to compute running decremented total, subtracting the previous cell

Question

This doesn't seem like it should be difficult. I'm keeping a running decremented total owed, where each line may apply some payment against the outstanding debt. Basically:

A: Outstanding Owed    |    B: Amount Applied    | Other Fields

R2: 5000               |       1000

R3: 4000               |        100

R4: 3900               |       XXYY

R5: A4 - B4            |       XXYY

But ARRAYFORMULA gives circular reference when applied.

Answer 1

Short answer

Add the following formula to A3

=ArrayFormula(
   A2
   - 
   MMULT(
     N(ROW(B2:B5)>=TRANSPOSE(ROW(B2:B5)))
     *
     Transpose(B2:B5),
     SIGN(ROW(B2:B5))
   )
 )

Explanation

To the initial value, A2, subtract the cumulative sum down column, also called running total. One way to do this with array formulas is using MMULT. The construct shown here of the cumulative sum down column was taken from the reference.

References

MMULT Usage by Adam Lusk

Answer 2

You can't use arrayformula. You need to use INDIRECT function. See: Google Sheets example