The potential damages caused by mal-functioning software to money eco-systems and the integrity of payment systems can be devastating to individuals and corporations. Lost or duplicate transactions, money-records that do not balance or that is out of sync with master records, pending transactions without information to resolve payments that have not cleared, slow (or absent) confirmation of payments and user-interfaces that display wrong or inaccurate information are just some of the things that can go wrong.
Agile is without doubt the best way to build software quickly and fast. It is the best way to ensure that results are seen quickly and to allow for less-costly adjustments early in the life-cycle. But, when building payment solutions, one probably needs an additional (traditional), robust quality assurance step prior to releasing it into the wild.