I would say it could be anything. That concept could be presented many different ways. For example a magical pouch (item), that replenishes its gp supply nightly (or increases it even), but only if the gp's are given away as charity and acts of kindness. Could lead to some fun plothooks. Thieves after the puoch and so forth, stealing it, only to discover that the pouch does nothing for greedy types, etc, etc, etc...and that is just one thought of the top of my head.
Nice idea, NCS