Analyse this small control file before applying the method to my larger file.
Group by the supplied category. Preserve unique record IDs.
Report the known subtotal, numeric count and unavailable count separately.
A blank amount is unknown; an explicit zero is a known number.
Show which source rows contribute to each answer. Stop if a record ID
is duplicated or missing, a group is outside the supplied set, or an
amount contains invalid text. Return the calculation method and actual file.

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Compare your result with my independently supplied expected totals and
counts. Then run these controlled changes one at a time: unavailable
becomes zero; duplicate ID; missing ID; invalid amount text; unexpected
group; and £10 becomes £20. Record the expected and observed behaviour.
Restore the original control data before saving the finished file.

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The method treated the blank amount as a known zero. Preserve its unknown
state and report it separately from the known subtotal. Show the affected
formula or calculation, rerun the original control and the blank-to-zero
change, and return the updated file. Do not hide the failure by changing
the expected answer or replacing invalid inputs with zero.
