This only holds if training such as huge expense, that the one time cost to train is such that A can never improve salaries. If B is paying so much more than A,

This only holds if training such as huge expense, that the one time cost to train is such that A can never improve salaries. If B is paying so much more than A, there has to be more than the training differentiating them. If that's all there is, then A can say "You're Salary is X during training because training costs Y, once training is done your salary is X + Y". Then the employee wins by getting paid during training, and the employer wins by retaining the employee. You're suggesting that B's salary Z is > X + Y, which could be possible due to the training infrastructure setup to train to a degree, but it's not like salary is the only lever that can be pulled. What seems more to be happening is that A is trying to say "Your salary is X during and after training" and then after training is done pocketing Y.