Well you are right. Any drop in Vet's salary doesn't count. Only a raise in salary is counted into the training exemption.
It's the players salary at the time you purchase them. It's also written in the news
Their logic is contradictory,
but not if you can guess what they meant:
Riviera had a salary of about $108K when I brought him a few seasons ago, when his salary drops to $50K in a few seasons time, would the $108K figure still be the figure that is used in calculations, even though his new current salary is only $50K?