Money in a computer is easy to get wrong. A binary floating-point number cannot hold 0.10 exactly, so a long list of small amounts can drift by a cent or two, and nobody can say where it went.
We store every amount as a whole number of the smallest unit, such as cents, and round only when a rate is applied, with the rounding rule written down. Totals then add up exactly, and a reversal removes exactly what was posted.