Remember the over-extension tax? The salary he had when he first joined your team is the salary you will always pay, until you sell him. For example, if I buy or draft a 2k salary player, and train him to 50k salary, I only pay 2k. However, if I sell him and buy him back, then I would pay 50k.
Not quite - his salary will still increase, so if you train a 2k player to 50k salary, he still costs you 50k per week. It's just that the overextension tax calculations will treat his salary as what his salary would have calculated to when you purchased/drafted him - which, of course for a draftee will be the same as his salary, but may be higher for a player who has received training since the salary update before you purchase him.