Lets say im a SL team with 250 k left on the cap this year

As it stands what stopping me offering him a six year contract at 250k a season.

and having a word with the player ...

"Look Benji after 3 years well sack you unfairly and you can claim unfair dismissal and get paid the rest plus costs so you'll really be getting 500k a season"

Would the sacked years still count on the cap?

If you were being very cute you'd just back load the deal.