Why payroll drifts, and what stops it

Ask where a payroll error came from and the answer is rarely the formula. It is that somebody typed a revised basic into the salary structure in March, and into the payroll sheet in April, and the two disagreed.

The fix is not more validation. It is having one place the figure lives, and having every other surface read it.

What that means in practice

A salary revision writes a revision row. The payroll run reads that row, computes the arrear itself, and marks it processed in the same transaction that writes the entries. There is no second place to type the number, so there is no second number.