Deferred money has been around for a while, the problem is that the players (and agents) have to agree to it. And they rarely agree to it because a dollar today is always worth more than a dollar tomorrow.
If you deferred 1M in salary from 2015 to now, the 1M you receive isn't as valuable in 2024 dollars as it was in 2025. 1M today is the equivalent of ~730k in 2015 dollars. So choosing to get paid later would have lost you nearly 25-30% of the value of that money. On the other hand, in order to get the value of 1M in 2015 dollars, you would need to sign a contract today worth about 1.3M.
By kicking the can down the road with deferral, he's getting the same amount of nominal dollars - "1 million" - but those one million dollars are worth far less than they were when he signed the deal because of inflation.
In the same way that, like, $10 could get you 5 McDonalds burgers in the mid 2000's but it's barely enough to get 2 today. Things are more expensive, money is depressed, you're functionally leaving money on the table by getting deferred payments.
So, why would a player chose to do that?
Now Jarvis is taking a minor loss, but this deferred money is less than 5% of this deal and the term he gets is probably worth it in his mind. But when you're talking about a star player, they could be leaving potentially tens of millions of dollars on the table in value by choosing to get paid later