I would ask myself if this condition (the field becoming negative) happens
very often. If it does not, maybe the best in performance terms would be
isolating the programs that allow this condition to happen and change them
On the other hand, a trigger such as the one you need is very simple, and
perhaps it would not have much impact in performance terms and (maybe) it
could be a better solution, depending to your business's needs.
IBM Certified Systems Expert — eServer i5 iSeries