BB-Charles wrote something about potential being measured as a combination of skills. Someone had asked whether potential was a limit on each individual skill or on more on the sum of skills. His reply was that it was based on the sum of skills, in qualitative terms. Salary is probably something similar to a weighted sum of skills.
But I'd think it would be messy to apply salary directly to training, unless you did something like using the ratio of a player's putative salary to his potential-based soft cap, and adjusted the rate of training, where the putative salary would be the salary a player would receive if he were getting a new contract.
So if a player were at 200% of his soft cap, his training rate might be set to 0, and if it were 101% it might be set to 99% of the rate based on everything else (age, height, trainer, minutes, skill, etc.). This would have the effect of slowing all skills development, but if you were broadening a player's skills the brake would not be applied as sharply since his putative salary would not be increasing as fast.